I'm not aware of any published, useable WAAS coverage in Europe. However, this doesn't of course prevent your GPS from being capable of receiving it. Most of the more recent GARMIN Panel-mounts are either WAAS-capable or WAAS-upgradeable.
However, the big buzz around WAAS is not to do with improved enroute navigation. Rather, the US has been trialling WAAS-GPS approaches - the key feature of these approaches being that they provide VNAV guidance and not just Lateral guidance.
The implication is that with WAAS available, pseudo-ILS GPS-based precision approaches will be available to normal Cat 1 minima in fairly short order.
Wait and see how long it takes the CAA to approve that
You can be sure though, that handhelds (to return to the original question) will not get that approval.
So the bottom line is that it is all a bit academic whether or not your handheld can or cannot receive WAAS. The primary advantage that WAAS is set to give, will not be authorised for handheld units.
2D