The whole use of your (Belgian) eID for online shopping and whatever idea has always been enormously short-sighted and needs to die in a fire. I’m Belgian and while giving the cops my eID is one thing (for them, it really is the same as a driver’s license), handing private entities a singular tracking identifier of me is something I will never submit to. Not that using it for tracking would be legal, but when did that ever stop anyone?
It’s a similar problem with OpenID. Usually when I post something somewhere with OpenID, you can follow it back to that identity. It’s not just the owner of the blog that can see who I am (or at least one identity of me), it’s whoever cares to crawl back down the link to my identity page. Which makes it a really good way to create targeted marketing user profiles.
Now @Aaron, there really is very little chance Belgian cops could get at much more information than American cops could using your driver’s license. The data collected from your eID, i.e. an identifier used to log you into that forum, lives in a private database that they simply don’t have access to. Even their access to government owned databases is in theory heavily regulated. I’ve heard of at least one example of a guy getting a not-so-friendly visit by the FBI after some anonymous comments on a forum, so you might want to watch what you say either way. Especially now that anti-terror laws mean you can be put away for years for what are basically thought crimes.
In practice, cops do often violate the restrictions based on them (most common example, opening up the files on celebrities that commit suicide). At least we know about it (access is logged meticulously), but unfortunately we don’t really do anything about it, which is a cautionary tale for anyone supporting giving the government (any government) more access to your data.