I don't think anyone needs to worry. iTMS has done in a week what the existing stores have done ever (in the UK). MyCokeMusic.com is part of OD2. I think it's all the others that need to panic.
As I keep saying, hopefully this will mean steps towards others ditching the WMA monopoly. iTunes is the only store available to Mac users. Of course, I'd always use iTMS, but it would be nice for better AAC support all around.