This mod will add a default avatar for members who have not selected an avatar. I haven't seen it released for vB3 Gold yet, so here it is ;)
Some styles code in the postbit and postbit_legacy templates are different. If you are having trouble, just attach your postbit code to a post and I'll sort it out for you.
The only bug I could see is if a member chooses not to view avatars, he will see the default avatar for everyone.