I always find it interesting when English speakers think Spaniards are 'lisping'. We have the same sound in English: cinco in Spain is pronounced like " thinko". I wonder if the same English speakers think we are lisping when we pronounce the th that way?
I think that English speakers know that Spaniards don't lisp. The word lisp+, though, describes a speech impediment that makes it impossible for a person to produce a sibilant sound; a +th will be substituted instead. As far as I know, there is no simple way to describe the Spanish pronunciation of the sounds that we expect to hear when we see words like cinco, soltero, or Zaragoza, so it's easier, although inaccurate, to describe the sound as a lisp.
Northamerican,
As a Spanish teacher I have heard many English speakers make fun of the sound (I use the 'lisp') doing their Yosemite Sam type accent and completely unaware of the fact that we have the sound. They seem at loss to extrapolate and recognize that the same sound is represented by different letters in both languages. As far as Latin Americans, they like to say things like theis and thiete (6 & 7) when making fun of the accent. BTW, the letter s is never pronounced that way, so tholtero doesn't exist.
As far as I know, there is no simple way to describe the Spanish pronunciation of the sounds
I easily say you pronounce it like the th in think.
I think that English speakers know that Spaniards don't lisp.
Supposedly the sound comes from the fact a king did in deed lisp and those who liked keeping their heads imitated him. So, many do believe it is a lisp.
I apologize for the mistake on "soltero." It's really just before e or i that the pronunciation of the s changes, isn't it?
As to that "lisping King" story, I thought that was disproved a long time ago, but I guess they still tell it.
NA,
The letter z in any combination and a c followed by an i or e produces the th sound. No other letter produces it, including the s.

[Seseo|lema.rae.es/dpd/srv/search?id=IIUwJDU07D6XC2xEky]
[Ceceo|lema.rae.es/dpd/srv/search?id=Cmi8vUAxBD6QBOs8iA]
[Pronunciation of letter S|lema.rae.es/dpd/srv/search?id=ZIsTSc1e2D6eQe60F6]
The last link at #16 reminds me that in Argentina you will hear people say that they don't speak español+, but rather +castellano argentino. And they suppress the s in "castellano" so that it sounds like the ch in the name of the composer J. S. Bach.

And they suppress the s in "castellano" so that it sounds like the ch in the name of the composer J. S. Bach.
Is it something similar to the way this former minister of the Government of Spain pronounces (sometimes) the S at the end of syllable (when it's followed by a consonant) in this video (take a look from 1:15 to 2:00 when he makes it several times)?