Apple's MobileMe service has been advertised as "Exchange for the rest of us" with support for push email and data. However, if you have MobileMe/.Mac account then you may have received an email this morning detailing a rocky transition from .Mac to MobileMe that clarifies not everything is pushed. Joel put together a post on Geek.com that has a copy of the email you may have received.
Email, contact, and calendar changes made on the iPhone or iPod touch are immediately synced, but changes made on a PC or Mac take up to 15 minutes to sync. You still can force an immediate sync manually. The word "push" will no longer be used unless the MobileMe team gets it working as originally planned. Many people are having issues getting things to sync completely so there are still some glaring growing pains and maybe Apple shouldn't have tried to launch so much (MobileMe, iPhone 2.0 software, and iPhone 3G) at one time.
The email also states that MobileMe subscriber's will get a 30-day extension to their service. Mine actually expires in about 20 days so I wonder if I'll get another 30 days to try it out since I haven't yet decided if I want to use it or continue with my hosted Exchange solution. I am leaning towards sticking with hosted Exchange since I use so many different mobile operating systems and the MobileMe service only works with my iPhone.