The Zune Pass will work on up to 3 computers and 3 devices, with "device" defined as a Zune player or Zune-enabled phone. I have it on a desktop, 2 notebooks, a ZuneHD, and a phone: thus, I can still add either a second player or phone. You can also replace a computer or device every 30 days. This really means that if I delete a third computer or device from my account, in 30 days, I can add a new one; so, I cannot be rotating multiple units in and out once I reach the 3+3 max. One other condition is that all units have to access the Zune account with and through the WinLive address linked to the pass. Thus, again, you and another user cannot access the Pass features (streaming and downloading, for instance), simultaneously, although, of course, you can play your entire "collection" (downloads (drm-protected and not) + rips) on all units at any time.
BTW, if you can spare the cash outlay, you can get an even better deal with a $150 annual pass that comes out to $12.50 a month. I figure that with my 10 credits, I'm paying $2-3 a month to rent 4000+ songs -and that's really an artificial ceiling that applies only to the ceilings on my 32gb ZuneHD and microSD expansion card. I'll soon be zooming past 32GBs on my computers and will have to be doing manual adds/syncs. Yes, I have a serious lifelong music habit. Except for my public library, I have NEVER had a better entertainment value. It was my Zune Pass that made me a ZuneHD and WP7 user.