Apple are deadly against hacking or modifying any model, whether it be the iPhone or iPod Touch. No-one has broke the iPod Touch ... yet. For now, people are making applications like
FLICK IM and
BEEJIVE to work with safari, and cloaking them to look like the iPhone's interface. So at the minute, no 3rd party 'download and run' apps available, but safari java and ajax scripts are available for tasks like instant messener and search engines.
Hope this helps you decide.