There are 5 new trailers on the Apple Quicktime trailer site. I opened 5 tabs and started opening each trailer in it's own tab. I assumed that they would each download (while I checked my email) because I'm on a cable ISP.
About 6 minutes later, when I came back to Safari - one tab had downloaded. I found that only the tab I was viewing had actually done anything - the other 4 were still at the start, but when I tabbed into each one, they would then start, but stop when I tabbed elsewhere. Is this normal/expected?
I've noticed this with some Flash animation sites, too, like Mark Fiore's great site. Unless the page is the front-most, loading of the multimedia seems to be stalled.
What's probably happening is that there's some kind of detection going on to determine if you're actually viewing the page, and won't "roll the tape" until someone's watching - meaning the viewer won't miss the start of the piece.
The technical reason has to do with threading, in that in tabs, all the tabs all use one thread. I'm not of the actual specific techincal details, but with only one thread, only the frontmost tab can load "interactive" content (eg. Flash, QuickTime), as CubaMark has explained well.
AFAIK there is no workaround if you still want to use tabs. However they load just fine if they are opened in new windows.