Be prepared for infrequent buses and almost no English spoken beyond Kerman.
Kerman (and the region east of it) is considered less secure than the rest of Iran due to petty crime, opium smuggling and the increased police/army presence. Things have much improved in the last decade, but the road from Bam to Zahedan – and especially from Zahedan to the Pakistan border – is still a concern.
Currently several foreign embassies advise against travel to Bam and the region further east of it. Ignoring this advice could negate your insurance, so it's important to check the latest situation carefully. Read the Thorn Tree (www.lonelyplanet.com/thorntree) for recent traveller reports.