Also confirming that this issue happens in 11.04. Seems to be specific to the synthetic NIC in Hyper-V - the legacy one might be OK. Previously, I'd gotten "around" this problem by installing WatchDog, and auto-rebooting if it couldn't ping our default gateway.. not a fix by any means.
edit #1 - nope, happens to the legacy NIC too.
edit #2 - Someone on the Fedora forums (http://forums.fedoraforum.org/archiv.../t-254272.html) suggested getting rid of the irqbalance package. Obviously there are repercussions of this if your VM has more than one vCPU.
sudo aptitude purge irqbalance