What does this have to do with your iPod Touch?
See this page: 'The UDID is a 40-character string unique to each iPhone/iPod touch. The UDID is needed for ad-hoc distribution applications. Typically you will need to e-mail the UDID to the application provider, so they can generate an ad-hoc mobile profile for your iPhone/iPod touch.'
Beta applications won't have Apple approval and so won't run on a tester's device: the UDID is used to get an authorisation from Apple to allow this. Whether there's any security issue I don't know, though I don't see how since even if someone obtains your number they can't use it to run an app on their device; and even if someone created a malicious app you would still have to install it (something you should always exercise caution about if the source isn't completely trusted).