A little more info
Push e-mail is polled continusously on the server-side for updates. MobileMe then "pushes" your new e-mail out to you. The Blackberry was revolutionary as a push e-mail device. Currently, most e-mail accounts on the iPhone and Touch are "pulled" e-mail accounts. This means that you set up a regular interval for the device to log on to your account and check for new e-mail.
MobileMe is an intergator service that allows you to set up your imap accounts online and have them all pushed to your device. Also, MobileMe provides the same instant "push" function for your calendar, contacts, and photos, syncing them wirelessly between your computers, website, and device. It is very similar to Microsoft Exchange.
The Touch probably does not benefit much from this since it doesn't have a constsant connection to MobileMe unless you are connected by wifi. That sort of defeats the purpose of an instant "pushed" account.
For those of us who use the iPhone as a smartphone, the benefits of exchange and MobileMe are significant. For example, if I have to sync by USB then my secretary won't have my updated info while I am on the road and we will frequently schedule on top of each other. MobileMe insures that all my calendars are synchronized all the time.