And people said market share didn't matter, that and the lack of flash support in iOS
If the beeb wanted to roll out to the largest number of people first then that is what they've done.
Android 2.2 covers a fair amount of smartphone market share (htc desire / hd, samsung galaxy s, nexus s etc) as well as all of the android based tablets.
I suspect the use of flash is due to the content already being in flash and that it scales nicely to any screen resolution.
Having just tried it on my galaxy it is very similar to the iplayer mobile website, however there is far more content avaliable from various programs i've searched i can watch every episode of the latest season and not just the most recent one.
However the download mode would be much appreciated, i'm guessing its the mobile providers that have blocked the usage as they all now have a 'fair usage' on there data plans which would be used up in a couple of hours of tv.