As yet another sign that Microsoft is hardly giving up on WM6.x devices, they have just announced the ability to update Mobile Outlook on WM6.1 devices.
Such an update is key for the hundreds of thousands of Windows phone users who are still using "legacy" devices such as the Treo line (750, 800w, Treo Pro; HTC Diamond, Mogul, etc.). While these devices are considered old by today's standards, no doubt many in enterprise still rely on them for daily use.
The update is mostly to make said devices compatible with Microsoft Exchange Server 2010, which is a major update to the popular infrastructure solution.
Threaded email with Exchange is supported (but not the cool, universal type found in some WM6.5.3 builds) Other enhancements include the ability to sync SMS messages, calendar lookup and enhanced voicemail (via Exchange).
Unfortunately, it is not obvious how regular customers can just upgrade via a .cab file, though we'll keep a lookout and keep you posted.
Update: In comments, Kareljack gives us this link for the .cab file update. But it is also noted that these new features only work with an Exchange 2010 account (either at your work or hosted).