You can get them off your phone while jailbroken, but I don't think it's been confirmed yet if you'll be able to load them back on iOS5, or if you'll only have the option to buy new tones from Apple. (I could be wrong; I haven't really been following the iOS 5 betas.)
At any rate, it's easy enough to save them now using a program like iPhonebrowser or iFunbox. Google around a bit and you'll find one. If they are tones you're activating in Winterboard, the files will be in Library>Themes>(Name of theme)>(File Name; will be something like sms-received1.caf) Once you navigate there you should be able to simply save them to your computer.