Tim,

That’s odd, it definitely shouldn’t ask you each time.

If you open your Aperture preferences, there’s an option to Reset All Warnings on the General tab. That is there to reset warnings that you’ve said “don’t warn me again” on, but maybe it’ll reset that, too. If not, try trashing the preferences, located at ~/Library/Preferences/com.apple.Aperture.plist. You can always move the prefs out, relaunch, say “no thanks”, quit and relaunch again—if the dialog is gone, then you know you solved it by removing preferences, and so then be sure to open the Preferences and reset everything else. If however this doesn’t fix it, go ahead and quit Aperture and put your old prefs file back in, so you don’t have to reset everything.

I’m not sure where else the MobileMe warning might be, so I’d have to dig around. Let me know if these suggestions work, and if not, I’ll see what else I can find.

Oh one more thing—make sure there’s no half set-up account in your Preferences under the Web tab.

-Joseph @ApertureExpert