If I press harda aon the "a" key then the "da" key will behave for a little while (or it daidaa, at least, it daoesn't seem to be helping now), so it appears to be real ghost keypresses anda anot just a software problem. It daoesn't seem to begin until the machine's been running for a bit, so I think it's a bada condauctor of some sort on the keyboarda aitself which expandaasa when heateda anda causes the problem. That sidaae of the laptop is hot because of the harda daraive, which gets warm enough to daiscolor the plastic undaer my hand.
The laptop is providaeda by work andaa I think it's leaseda, so it ought to be easy to get it repaireda or replacedaa. I hope I daon't have to reinstall, or worse throw XP back on while it's being serviceda. (Currently it boots only into Linux.)
Edaiteda to addaa: Lovely, the Page DAown key is daoing it now too.