The really interesting part is if they'll try and continue to restrict resales in North America while being forced to allow them in Europe.
Speaking in general, I'm sure most software publishers will, at least for now; the non-transferable-license model has been upheld pretty well by our courts. Speaking specifically of Steam, and in the long term, are trickier prospects.
Double-edged sword. I'd rather DCD organizations found a way to facilitate this themselves (btw I still have that spare Indie Bundle key) rather than being mandated to do it before they were technologically prepared for it.
The technology's the easy part. Hell, I'm not sure the publishers have to do anything at all except stop suing people for reselling licenses.
Valve's going to have to walk a tightrope now between allowing transfer licenses and not making publishers flee like rats, and the only real current solution is more restrictive DRM.
For certain values of "real".
As far as publishers fleeing, well, Valve doesn't occupy as privileged a position in game distribution as Apple does in music, but it's pretty close. EA's in a position to tell Valve to fuck off, but most other publishers aren't -- and the ones that are would probably be looking at EA as an alternative.
And again, that's assuming Valve changes anything at all stateside, which they're not obligated to do.
The EU's another story, and given that this is an across-the-board ruling, there's no reason for publishers to flee Valve for any other distributor at all, because they're ALL going to be subject to the same requirements.