I think Nokia's goal was less to expand the IT market by adding WWAN capabilities than to establish a better position (in the US, and in WiMAX) by delivering a unique and desirable product at network launch. (Some device has be the flagship at network launch, why not a Nokia tablet?) Adding a UMTS/HSPA device wouldn't have gained them a similar position.

However, while Xohm has suffered its delays, the N810 has aged, and it's geek-magnet slot is being squeezed by competition from above (Netbooks/MIDs) and below (smartphones). This makes the positional gain less sure, and potentially even risks brand damage by association with Xohm, now viewed as a flop. It also limits expected sales, even if it does get a toehold they can (try to) leverage with the next WiMAX device.

So I think the reason it proves to have been the wrong choice is less because of poor coverage, which any launch device will suffer, and which was planned for, and more because the Xohm delays pushed it out toward the end of the N810's lifespan.

There's been various speculation about the intervening device Nokia "should have" released in time for Christmas 2008, but if one had existed, I'm not sure a Xohm version at release wouldn't still have been a good choice. It's just that you can't carve the foothold Nokia wanted with last year's tech. (And, of course, since Xohm has had so much trouble, I'd need some strong assurance to bet on their rollout going well from here forward.)