The My-CAST version we have been waiting for just got put in the market. $3.99...best weather app ever!
Rainy Days is an awesome free app for radar (straight NWS feeds in the US (what all those other products are based on)). I haven't used My-Cast to compare it (and probably wont)...
Anyone try loading Weather Underground for iPhone in their browser? I don't have my DInc yet, so I can't verify, but I really liked this mobile page on my iTouch. Had zoomable looping radar, loaded personal weather stations (including the one in my backyard!) and everything.
For what it's worth, the best mobile radar app out there is RadarScope ($9.99), but alas, it's iPhone only for now. Lots of chasers use it, and there are extensions to layer your position on the screen. I only wish it would be released for Android.