I think you can assume that it sent to the names above the first one. I tried the same thing with a smaller list (like 60 or so names), and what I did was select all the names below the one that appeared when it said "cannot send" (including the first one one listed), then paste those into the address field of the original message (deleting the names above them, assuming Mail had actually sent to them), and hit send again. This time they all went through. I've since gotten a reply from one of the recipients in the first group, so I'm assuming from that that the first group did get the message. Maybe you could put your own email address in the first group to see if it sends to you.
Oh, and thanks for explaining why I can't find the original message in the Sent mailbox -- I didn't realize till I read your post that Mail doesn't put a copy of the message into the Sent box unless it sends with no errors. So if you need access to your original message after sending, better be sure to copy yourself on the message so you'll have a copy. Luckily, when I needed to resend the original to some new addresses, I had a copy that was contained in the reply from the guy in the first group; good thing he didn't delete the original!
I think Mail still has some growing pains that need working out, and that inability to send to large groups is one of them. But I still use it instead of Eudora because of its tight integration with the Apple apps.
bc