I have preferred Debian and Ubuntu over RHEL mostly because their package collection was much wider. Thus at work I wanted to go with Ubuntu LTS as deal between Ubuntu's bleeding-edge and a more conservative approach of package versions while Debian Sqeeeze is not yet out. Happy thought I with the arrival of Microsoft hv-Moduls in the staging area things would be easy now. - Especially after 2.6.35 when MS included virtual SMP and shutdown integration. I will tell you later about more details but at the moment I can only give you this advice:
Either you don't see error messages when using Ubuntu on Hyper-V during boot and during usage - and you are lucky - or you can see dieing network drivers and block devices that just kick out and will not be accessible until a full reboot of the respective VM. MS tested its 2.1 version against RHEL 5 and some SLES versions which are enterprise linux with mostly older kernels. They work, at least MS has certified them they should and I know people who don't see problems in bigger setups. MS' Linux IC 2.1 are yet the way also when using CentOS.
For the Yet I have to say: hv is fine for you nerds and those interested in experimenting. For some it may work on non-Enterprise distributions, but be prepared for surprises.
I will have to switch all my Linux-on-Hyper-V to CentOS or RHEL yet, neither was I succuessfull under R1 or R2 with Ubuntu LTS or Debian squeeze and lenny. I had lots of enthusiasm for Microsofts hv-Module and hoped it was in better shape. But when I just need a server to work, I will not touch a staging-driver anymore for something critical, it hurt and cost me enough of time.
For the Debian guy in me it's like changing from his beloved comfortable car with apt and big bazaar of packages made by the Debian developers to a "also well working" but less attractive Vokswagen transporter: It's older, but at least: It works but almost every extra tool will need to be imported from outside (for example from remirepo). - But in the end it's solidity that matters.
I recommend you to read the comments here for example about dying Network interfaces: http://www.panterlo.com/2010/10/10/ubuntu-10-10-and-hyper-v-r2