I could certainly comment on those issues.  I'll even include a basic thought process here for the first question to provide some evidence of my qualifications.  Payment of payroll in Bitcoin would appear to require an immediate valuation of the compensation on the date paid.  The source of the valuation is probably a bit flexible, but consistency would likely be the rule that would be most important.  Thus if you valued with Mt.Gox once, you should probably do so every time (unless you can document a very specific reason why you change, such as the site is not available on the date).  Might make sense to build a formula that does a weighted average of multiple sources just to try and smooth out some volatility.

You would report the dollar-value estimate as compensation and withhold an amount of cash valued at the required tax table rate.  If you're trying to pay only in Bitcoin though (no cash portion of payment), you'd need essentially build an employment agreement that included an "employer buy back" of the right amount of Bitcoin to withhold for the tax.  I.e., employee is to be paid 10 bitcoin, withholding requirement is 1 BTC, you agree to buy (for cash) the one bitcoin back immediately at payroll, thus providing sufficient cash to cover the employee's withholding. 

The amounts granted as compensation and withheld would then feed directly into the appropriate W-2.  Of course, this is all assuming a W-2 employee, if you're looking at 1099s it would be far simpler.