As a Scrum Coach, I get asked these questions all the time.
Note that in the post below, if it ends up on the PBL then you would assign whatever units you use to estimate PBI"s, but if on SBL, then whatever units you estimate for items on the Sprint Backlog. http://ScrumCrazy.com/bugs
Note that the above approach is just one approach. Scrum simply dictates that PBI's and items on the Sprint Backlogs have some way of being estimated. It is up to the Scrum Team to decide how that is done.