Tablet PC Talk has put together a huge chart, the Tablet PC 2004-2005 Quick Comparison:
Find the latest Tablet PCs announced since September 1, 2003. This comparison allows you to sort or reverse sort the table of Tablet PCs by clicking on the links at the top of the columns so you can easily identify the Tablet PC that supports the attributes you want. Also, this table will print in landscape mode nicely as well. For the detailed comparison see the Tablet PC Talk – 2004 Hardware Comparison
Quite a few convertible ultraportables are listed.