I have seen these types of cards for quite a while now, but never in the UK or USA.
I have been hoping for a 3G Compact Flash card
http://www.3g.co.uk/PR/Nov2003/6150.htm but you try to find one in the UK.
I have read people buying them in Japan, they even sold one as part of a Vodafone contract, and apparently they have worked in PDA's and Tablet PC's, albeit only with a Vodafone Sim card. However I don't know what the battery usage implications are.
If anyone has had any experience of these I would also be very interested.