OK, so perhaps you can't email into WordPress.com, but I've found a possible solution. There's a tool called Pocket SharpMT, which seems to speak to WordPress. I'm posting from it now. Only problem so far is that it doesn't seem too keen on working with my PDA's keyboard: not that it isn't working, but I can't see what I'm typing.

UPDATE (23.1.06): I've spotted a few hits to this item today; wonder why it's suddenly of interest? Anyway, my new recommendation is the Windows Mobile version of DopplerRadio, currently in beta but working very well as a blogging client.