I sended a large amount of undetected viruses long ago,and they are still undetected. Someone also sent me an archive full of viruses and like 30 of them were not detected. Also lots of trojans (reports from my friends and their relatives) were not detected (sorry but i don't have any sample of it,because they deleted them).
I think you should add definitions for every virus even if its very unimportant. Than those peoples scan with some other scanner and they see that they still have nasty things (after avast! scan) on their hard disk and avast! is getting bad reputation with every such event.
I understand how it's with these things,but most peoples don't.
If you need those undetected ones,i can resend them,because i'm still storing them. Also improving some form of heuristic detection for Standard Shield or better generic detection would be good thing to impliment (soon) to keep step with the others (BitDefender and NOD32 are very good in this field)
Thanks