new UM card with +1/+44 dual number soon

[inquisitor]

Originally Posted by Stu

UM should allow you to have both numbers active at once and charge a surcharge to forward the US number to the +44 when it is out of the country. SMS forwarding would also be great.

With 44/1 you are definitely available on both numbers simultaneously no matter where you are. So there's no need for forwarding, as that's a basic feature of the service.
However there's a difference in pricing: When you are being called at the US number, you'll always pay a call setup-fee of € 0.19 + at least € 0.19/min, no matter where you are, while calls to your British number are free in many (especially European) countries (except for the call setup-fee of € 0.19).
So it's still 63% cheaper to get a (free) US DID-number from some VoIP-provider and forward incoming calls through poivy.com for € 0.07/min to your Britsh UM-number, than using UM's US number, for which € 0.19/min are charged when receiving calls.
For those, who are able of setting up such VoIP-forwarding, UM' 44/1 doesn't bring any advantage, except for being able of receiving SMS at a US number.
In my eyes 44/1 is rather interesting for non-Americans, who regularly travel to the US and want exceptional coverage (in the US 44/1 roams on T-Mobile and AT&T) while keeping a US-number for the long term.