I used to have a Zaurus sl-5500. I think Nokia are a lot more involved than Sharp ever were. In the UK Sharp only launched one device (excluding dev 5000), if memory serves me right, they only gave one update and didn't bother officially providing version 3 ROM, you had to get it from the US site and ignore the warning about being suitable for US models only. The support for the Zaurus outside of Japan quickly died and I think that meant for many developers a sign that the future was limited. Nokia on the other hand have been around longer than a year and still going strong, with a new device launched. There appears to be positive moves to improve on the software with recruitment still ongoing.

My only concern with Nokia is that there seems to be more hardware quality issues than you used to see with Sharp devices. WSOD on the 770 and what seems more common touchscreen issues on the N800.

As for Nokia staff helping out in the forums, I've found that they tend to post to the maemo mailing lists or use of blogs.

Rich