For folks on Cingular service with corporate Exchange servers - the new Cingular Xpress Mail service works well with treo. More info available here:

This requires a little setup by your IT folks to get it working initially (really just to open communication between your Exchange servers and the Seven Xpress Mail service - no new corporate servers or apps required), but it's worth it - since it provides a relatively low cost way to directly tap into Exchange from WAP phones and PDAs.

Pros vs. treo Mail:
* Doesn't require a desktop agent to be running. So if you travel with your treo AND your laptop, you can still get access to your corporate email without finding a wireline somewhere.
* Provides access to mail, calendar, tasks in Exchange - not just email.
* The interface for the treo utilizes Blazer and thus is "semi-graphical" with fonts, bolding, etc. so it's much better than plain old WAP.
* Fairly bulletproof in my trials so far (my company has been doing a trial of this for the past few weeks).

Cons vs. treo Mail:
* This is NOT a synchronization solution - you're composing, reading and replying to mail "online" - which means (a) you can't do it on a plane and (b) you're eating minutes to do it (which is why the carriers love this solution).
* Because it's online (via Blazer) it's not nearly as responsive as the local, synchronized mail apps on the treo (like treo Mail, Eudora, Palm Mail, Aileron, Basejet, etc.)

Overall, I like the combination of this plus treo Mail as long as my company is paying for both - But the ultimate would be a treo Mail service that provided a direct connection to the Exchange server without the desktop agent (and of course GPRS connectivity, push, configurable SMS email summary notification, etc.). I know Visto sells a corporate version of treo Mail - but I want an easily deployed version like Seven's that doesn't require new infrastructure in my company.