Does jailbreaking an iPod touch delete all previous data?

Your answer: no (tentative) meaning no UNLESS you screw something up, really bad, so if you can do it without getting any serious errors then you should be fine, but any SERIOUS errors (not like they happen often unless you have no idea what you're doing) and you'll have to do a full restore from iTunes, which WILL erase all of your apps, settings, etc.

SO your answer (in short form) is this:

No, unless you're really really stupid.