Pricing will be deciding factor
It is interesting to see that they'll be selling these UMPCs in supermarkets to increase awareness, but I think they will be severely limited by that price.
I am definitely in the target market for these devices I own a high spec PC, my wife has a high performance laptop, my phone is an all singing/dancing Window Mobile device etc., and an UMPC is definitely a desirable item for me.
But they are only ever something I want not something I even remotely need and at £800 ($1600-odd) I can't justify buying one. I think until they fall to the £200-£400 range which is comparable to a low priced laptop it will only be an oddity for the rich geek and show-off city-boy.