This is a big feature when it comes to putting Ubuntu onto tablets. Currently, Netbook edition works great for that purpose and the pen digitiser is perfect, but the handwriting would be a real dealmaker (especially for my business - we could actually move to Linux) to compete with the Windows one.
CellWriter exists, but that only handles character and keyboard input (but I don't know about multitouch on the keyboard). It also needs to handle print and cursive, because character mode can be slow and uncomfortable (unless you're writing passwords). Lastly, CellWriter needs to have some default letter shapes rather than having to be trained from the start.
There is a software package called MyScript (by Vision Objects) that handles all four modes (keyboard, character, print, cursive) plus calculator and fullscreen, but it's only free as a trial. Still, it would be nice to see it in the For Purchase section and the trial in the free section of the Software Centre.
The only other ones are for Chinese/Japanese/Korean characters.
What would really make a difference for us is the integration of some formal API with the OS that can automatically activate when running on a tablet to pass ink data to whatever recognition system is installed, and have something available (however rudimentary) to use it.