If you've noticed that the next Ubuntu Server version (10.4, Lucid Lynx) has the Hyper-V kernel modules packaged, alebit in drivers/staging, I'd suggest not dist-upgrade'ing even your development servers for the moment. The reason is simply that you need to devote time to ensuring that the kernel modules will continue to work with each kernel version - right now you can't seem to rely on the modules actually loading successfully from the corresponding /lib/modules/2.6.*/kernel/drivers/staging/hv directory. Which isn't a problem, provided that you have the time to deal with it.
The long and short of it is that if you're currently looking to use any flavour of Linux under Hyper-V the “old” rules still apply;Use the legacy network adapterSet static MAC addresses under the VM settings (unless you want to faff with udev)and learn to live with the performance penalty