Big revision recently to KB.810489. This is the KB that talks about the MTA function and how it can or can’t be disabled in various scenarios.
The core change is that if you’ve got super-large Exchange clusters (>4 nodes) in a native-mode administrative group (and do don’t have MTA-dependent foreign connectors, etc), you may want to disable the MTA service. You can do this on non-clusters too, of course, but it’s really more interesting in the large cluster case where all of the EVS in the cluster utilize a single MTA and may lead to event log errors and performance bottle-necks. Disabling the MTA can’t be done in Exchange 2000, as move-mailbox is still dependent on the MTA… but in Exchange 2003 it’s now possible and supported to disable/remove the MTA! Have a look at the KB for the details.
Update Sep 8, 2005: Mark has blogged with a bunch more detail about "disabling the MTA" scenarios. His post covers more than just the cluster case, if you're interested.