WebOS was fun to develop for (HTML and JavaScript!), but had only 1% of the market. WP7 looks more like the two market leaders (as in "less fun") but has about 3% of the market. Looks like an improvement.

I'd say it's worth a try, but, so far, Microsoft hasn't made contact.

If you already have iOS and Android covered, WP7 is worth a shot, but I wouldn't ignore RIM as a viable third player - their phones appear to be making inroads into the low-end and that segment is huge.