I just read news of an 11-hour outage in RIM's Blackberry infrastructure on ZDNet - ouch. Not only did email stop flowing to the devices during the outage, but the backlog of mail which built up is taking time to clear.
Without wishing to gloat (really), users of Windows Mobile devices for push email wouldn't suffer something like this (with the possible exception of their mobile carrier having a major network outage, which would affect Blackberry users as well and would be unlikely to last so long). Once you've deployed a real mobile mail solution, having any kind of serious outage is a worrying thing - especially if users are giving up laptops in order to rely on their mobile devices...
There are some architectural documents which outline the approach to using Windows Mobile and Exchange - such as the one in the Deployment Guide.
If you're interested in how Direct Push works, you'd do well to check out these posts on the Exchange Team blog too: