I wounder if my Nexus One has support for EGNOS? to get better GPS accuracy. If I check the GPS Accuracy value, it's sometimes 2 or 3m which is pretty good.
I'm going to say no.
From the article you linked, EGNOS is primarily designed for airborne use and has limited ground applications.
EGNOS looks to be designed more for commercial applications (e.g. embedded navigation systems in vehicles, airline, etc.), not so much consumer devices
Nexus One's GPS is provided from the QualCom QSD8250 "SnapDragon" chip (specs). The specs for that chip do not mention any WAAS capabilities. Furthermore, there aren't any Qualcom chips listed on the EGNOS registered devices list.
If I'm wrong about the chip, then it should just be a matter of software to enable it. I haven't seen anything like that in the Android source, but that doesn't mean it isn't hiding there or in some third party bolt-on.
What the Nexus DOES use to improve both response time and accurace is A-GPS (Assisted GPS). This basically just means it us using the presence of other location clues (cellular towers, wifi networks, etc) to help triangulate a better/faster position.
You can see this in action with the Google Maps application. If you have GPS enabled and WiFi disabled and open the maps application, it will prompt you to turn on WiFi, even if you're not connected to a network. It uses a location database of wifi hotspots to help get a fix on your position, similar to Skyhook
There is actually an EGNOS test application available from the commission. It can be found at: http://www.gsa.europa.eu/go/news/egnos-gets-an-invite-to-your-smartphone. It's not available through the market, but it does work on the HTC desire (so likely on nexus as well)