for new Developer = WP7 is the best place to start more potential sales.
for an existing and establish DEV = Android+Iphone
reason WP is good for new devs with new app/games , is that it has WAY more potential to get attention then on Iphone/android with the 50000000000apps , it just gets lost in the mess.
they did a test on this with releasing the same app on all 3 platform , and WP app was making the most $$$ cause it was getting WAY more attention due to low amount of total apps in the marketplace.
XBL games on WP = like 80 games? takes you 2 sec to see them all , hence = more potential. unless it already has a HUGE FAN BASE!
so in the End Angry bird not coming on WP right away makes SOME sens . but completely ignoring it is stupid cause they can still make millions off of it.
specially at 2,99$ a pop on WP compare to the 0,99$ on Iphone.