Entering US for 90 days without an exit flight

as long as my 90 day tourist visa allows

There is no "90 day tourist visa." The Visa Waiver Program allows entry up to 90 days, and is a waiver of a visa. A tourist visa (B-2) allows entry up to 180 days, and does not require an onward ticket to enter.

My question is will this raise concern at the border?

If you have the original ticket, you should meet the requirements and be ok.
However, they can ask about your plans (and often do), and if things don't agree, you could be in trouble.

If you enter the US by air under the Visa Waiver Program then you need to show return or onward tickets. They do not often ask to see that at US immigration but the airlines will often want to see it before they will let you board the flight to the US because they are on the hook if you are denied entry to the US. It sounds like you already have a return ticket, just that you do not intend to use it, so that should not be a problem with your airline. If you are admitted to the US under the VWP then that is invariably for 90 days, it is not that if your return ticket is in 2 weeks they will admit you only for 2 weeks and there is no need that the return ticket that you had at entry needs to be the ticket that you actually use to exit the US. If you are planning to stay in the US for 90 days then you should expect questions about how you will support yourself for that period and why you want to stay that long. Uncertainty and inexact plans will raise questions with US immigration, you would be wiser to have a fairly exact plan and then modify it later.

The US is finally coming up with better explanations of what you need in order to enter under the Visa Waiver Program (VWP)

1. If you are entering by air or sea, you must have ESTA pre-approval. ESTA is not how you enter the US; it is a pre-approval program that verifies tat you appear to be eligible to enter under the VWP, and that you are not on some list of undesirable people. Your airline will not let you board without it. ESTA approval does not guarantee that you will be admitted it other US. That is always at the discretion of the immigration official. You do not need ESTA approval if you arrive by land--car, bus or train from Canada or Mexico; ferry from Vancouver or Victoria, British Columbia, and Washington state.

2. You must satisfy the immigration official that you plan to leave the US within 90 days (or whatever period of entry you are given.) This doesn't have to be a piece of paper, but, if asked, you must be able to give good reasons why you plan to go back home or move on to another country.

3. You must be in possession of a return or onward ticket that takes you to some place that is not Canada, Mexico, or a Caribbean Island (unless you are a permanent resident there.) Officials do not always ask to see proof of the ticket, but the requirement is still there.

• If the flight or ship departs from the US, your ticket must have you departing within 90 days.
• If the onward flight or ship departs from Canada, Mexico or a Caribbean Island, you must have a ticket for every part of the trip. For example, if you are flying from Montreal to the UK, you must also have plane, bus or train tickets that take you to Montreal when you enter the US. They won't accept "we plan to drive to Montreal." Your ticket to the UK (or wherever) can be dated later than 90 days, but you must show that you are leaving the US within 90 days, and that you have a ticket that has you eventually leaving North America.
  If your flight departs from Canada, Mexico or a Caribbean Island within 90 days, the official may or may not ask for tickets for the other parts of the trip

(Your airline or ship company must have signed an agreement with the US about VWP travel. This does not matter for most travelers, so I'm not giving details. Just don't plan on using the VWP if you arrive by private boat or plane.)

@nutraxfornerves thanks for that. Getting a clearer picture now.

One scenario i've been thinking of is getting a flight exiting the US to Mexico (City), from where I intend to continue travelling down through Central America. This seems to satisfy point 2) but contradicts point 3). Or another example, what if I wanted to fly to Canada, before leaving Canada and flying to Europe (for arguments sake).

I guess the idea of 3) is to stop you from entering a bordering country, only to return back in for another 90 days (?) but if your intention when leaving is to travel onwards then are Canada and Mexico still not legitimate exit countries?

Thanks :)

OP, your further questions are answered in this thread.

#4 the way to make it clear is to understand that there are two separate requirements, firstly to show a ticket leaving the US within 90 days and secondly to show a ticket leaving North America sometime. Many people make the mistake of conflating these two requirements into a single requirement show a ticket leaving North America within 90 days but that is NOT correct. Also the ticket to leave the US within 90 days does not have to be an air ticket, a bus ticket which you can book over the internet will do fine, a private car is not acceptable though.

Thanks.

Ok, so from what I understand now, if I wanted to be sure of satisfying the airlines and border control for a journey of either:

US > Mexico > Central America or US > Canada > Europe/Asia/wherever

i'd need to show tickets for both US > Mexico/Canada (an exit before the end of the 90 days) plus the tickets showing me departing Mexico/Canada to my onward destination, wherever that may be.

And these tickets need to be purchased before I first arrive in America.

Is that all correct? :/

#7 Yes that is correct, you need a ticket leaving the US and a ticket leaving Mexico/Canada. You do NOT need to have domestic tickets in the US or in Mexico/Canada to show you are getting from your entry point to your exit point.