Apple released their latest iTunes version 10 yesterday after the Apple Media Event. As a jailbreaker, you should be wary of updating any new firmware or iTunes update since it could break your jailbreak/unlock or both.
It is usually a wise idea to wait for the Dev Team (the jailbreak masterminds) to advise if it is safe to upgrade.
And MuscleNerd from the Dev team has spoken!
It is safe for jailbreakers to upgrade to iTunes 10 without any worries. However, Apple is poised to released iOS 4.1 update next week and there is no current jailbreak available for it. So, DO NOT UPDATE when that is released!

