Getting your priorities right … P0, P1, P2, …

Since I have started working at Microsoft I have had to absorb and remember a forest of acronyms and when it comes to assigning priorities to work items, issues or bugs, we encounter the P? acronym. BugHere is a short summary of the priorities we deal with:

P0 Do not do anything else, this is a blocker. You can think of the Lancaster bomber flying over the English channel with 4 engines on fire … getting the fires out is a P0 priority.
P1 Must be fixed.
P2 Should be fixed, time and resource permitting.
P3 Might get fixed.
P4 Noted … this is one step away from the black hole.

What priorities are you using? I would be interested to see how others are managing their priorities within Team Foundation Server (TFS) and other ALM environments.