Well, the target="_blank" attribute being not XHTML compliant is only half the truth.
This only affects XHTML 1.0 strict and XHTML 1.1, whereas it is perfectly legal in XHTML 1.0 transitional and probably will be in XHTML 2. It might even find its way back into XHTML 1.1, see this article for more information (and a solution to put it back into the DTD):
for more information.
Thanks for the Article about how to put it back into the current dialog, I'll give that a try.
btw: What's up with the link editor in this forum? It only gives me web.config errors since a couple of days, and I have to copy the link to Word and back to make it clickable. (Firefox 3.0.5)