I have found myself re-using 66 blocks more than I would like to admit, I found that a nice touch to clean them up is to use a piece of fine sandpaper to clean the plastic frame of all the writing that happens on them, after that they almost look new.
[This message has been edited by mkoloj (edited 02-23-2005).]
We use a circuit board cleaner that will remove sharpie markings without any harm to the plastic. Works well for removing sharpie from wall plates too. We bought a case of the stuff maybe 6 years ago and it's since probably been banned on 20 countries because it's CFC based. I'm sure though that they have an ozone frendly replacement though I hope it will work as well.
Garden variety lacquer thinner ($3 a gallon) will take Sharpie off. Just be sure to test it on a small corner of plastics before you gunk it. It will eat some of them. It doesn't seem to hurt the real Telco 66 blocks.
Trichlorethylene and trichlorethane are both pretty much banned because of a number of heath and pollution issues. Not the least of which is they break down to phosgene gas when heated. (also liver damage and such) It does knock stuff like Sharpie pen off tho, just don't try to put it in a styrofoam cup.