After updating to the qemu-kvm packages, I don't seem to get working networking in my guests either if I use the usermode networking or with -net nic -net tap. The network device shows up, but no apparent traffic and no dhcp lease.
Does regular non-usermode networking work correctly ? (eg tap based)
As mentioned initially, nope.
What guest OS are you running ? I've just started an install of F10 x86_64 inside a F11 hosted guest, with qemu-0.10-0.9.kvm20090310git.fc11.x86_64. NetworkManager configured the NIC with DHCP and downloaded anaconda's stage2 image without apparent trouble.
Hmm, actually it seems to be slightly random, another guest install got stuck 30% of the way through grabbing stage2, and another on 90%. Guess I was lucky the first time.
I just saw that there's an update in today's rawhide for -0.9.kvm20090310, so my box is currently churning away at updates... this was with what was there last week (qemu-0.10-0.4.kvm20090303git.fc11 I think). And I tried both with rawhide and F10 guests (livecds)
-0.9.kvm20090310git definitely seems better for me, at least in terms of what I was initially seeing.