A couple of somewhat OT points to consider since you're still in the planning stage--
SBS2008's native backup is incredibly efficient, and IME hands-free reliable. It's reporting (through SBS) and granularity (little provided or needed) are sufficient for many small businesses. Forget everything you know about NTBACKUP; this is a whole new animal. Only meaningful shortcoming I can think of with SBS backup vs. BEWS is there's no GRT for the mailbox. You have to restore the whole database. However, there's an inexpensive add-on that gives you a full-blown UI and reporting for it if you want it, and it offers GRT-type functionality as an option. SBS Backups are hard-disk only, but with Hyper-V, that's going to be a universal issue anyway. But you'll probably get months or even years of backups on a couple of 2TB external USB or Firewire drives, because SBS Backup backs up only changed clusters after the first backup, similar to Shadow Copies. Only reason to use tape these days is for archive backups. If that's not an issue for you, I'd think twice before using BEWS or any other 3rd-party backup in an SBS 2008 system.
Also, whether SBS or a "normal" domain, make sure you have at least one DC running on physical hardware, or when your Hyper-V host boots, there will be no DCs available until the guest boots. That's not a good config. Off-hand, I can't recall MS's support policy regarding SBS in a Hyper-V guest, but that's also something to research if you haven't already done so.
