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You are free to upgrade to iOS 4 the day it comes out if you want to. You'd lose your jailbreak of course and all the benefits that go with it but being jail broken now will not prevent you from upgrading later.
Personally, I'm not excited about OS4 in the slightest, all the major improvements I.e. Multitasking have been available to jailbreakers for ages. If you want multitasking now, grab backgrounder from Cydia, works great on iPad.