They seem to have disabled it, and probably with reason. I had AOL Instant Messenger running (one of my friends uses it - I hate it), and it disconnected. So while in FOT, I got switched out to a dialog asking if I wanted to reconnect to AIM. I said no, and went back to FOT. Then for whatever reason, AIM popped up another dialog and I got switched out of FOT again. I shut down AIM this time and went back to FOT. I had just finished a nasty battle with 5 lifter bots on mission 13 (Great Bend), and about 10 seconds after coming back to FOT after AIM the second time, FOT simply shut down. No message or anything - just gone!
Now regardless of whether or not you think ALT+TAB should work, it appears that FOT may not like being suspended and resumed in this manner. This is, for the record, the FIRST (WOW!) time that I had FOT crash, and I can't help but think the AIM deal was the cause. So feel free to try some hack to get it to work, but my guess is that unless they spend some time patching the game to make it "multi-task" better, you'll probably experiences crashes much more frequently if you leave FOT and go to another application. Just my thoughts on the matter.