Google is changing its policy for storing your personal search information data: Google to anonymize user data | The Register
All search sites do the same, MS doesn't state what and how long it holds data and AOL holds the your user information for two months. Its been a while since I read this so I hope I have the facts correct.
Not sure what they do with deleted email/talk messages, if they are ever really deleted. I doubt anyone other than the government could get it from google. If you're that worried you shouldn't be using IM or email.
Anyway, as to your problem, as dumb as this may sound - if you're that worried something your sending may be a problem if you're company gets it, don't use your work issued blackberry to send it. I wouldn't trust the IT Department and you don't know if that policy will every change. The beauty of the BB is it is a centrally managed device, the policy could change anytime and you'd never even know it. I have no idea what version of BES my company is running and no clue what it can and can't do.