not sure if this is the cause of your problem or not, but Apple recently did an upgrade to MobileMe calendar service. If you have an iPad on OS 3.2 *and* you're using the upgraded calendar service, then you need to set up your iPad slightly differently than your iPhone to sync calendars. Since you recently signed up for MobileMe, I'm wondering if you got the upgraded calendars by default...
Go to your Mac and access your MobileMe account via the web browser. then take a look at
this Apple support page and you'll find instructions there "How can I tell if I'm using the new MobileMe Calendar or not?"
If you are using the new version of Calendar, then use
these instructions to set up MobileMe Calendar sync on your iPad. Basically you need to set up a new CalDAV account on your iPad - this is separate from your regular MobileMe account setting where you sync contacts, mail, bookmarks, etc. You should probably first turn OFF calendar sync on your iPad in your regular MobileMe Settings before turning on the new CalDAV account.
if you're not already on the new version of calendars, then something else must be going on so disregard the above info.
hope this helps.
EDIT: ** note: I think this separate CalDAV account requirement might go away when OS 4.2 is released for the iPad (not sure, but just guessing that since you don't need to do this for iPhone on 4.1). it seems 4.2 release could be any day now, so perhaps if you just wait a bit maybe it will work by itself when you update to 4.2??