This happens only with HandcentSMS or also with your stock messaging app? I had the same problem but only with HandcentSMS, the thing was it'd still be sending the message out an everytime I hit retry I ended up sending a duplicate message. Eventually the notifications became annoying and I switched to ChompSMS, haven't had a problem since.
I'm assuming it's solely a HandcentSMS problem an you just messed with your stock messaging options to see if that was the issue? Let me know.
It's something with the early 2.1 update on the Eris, it isn't compatible with something in the app, I'm not completely phone savvy so I'm not very technical with my terms but you get the idea.