In this Channel9 video we have Peter Torr, from the Windows Phone Application Platform Team, talking about how multi-tasking in Windows Phone 7.5 will work.
He mentions 5 apps will be held in memory at the same time, and if memory is needed the last used app will be terminated.
Regarding background agents, he notes most are only allowed to run 25 seconds every half an hour, and use only 6 MB of RAM and much less than 100% of processor usage, ensuring Windows Phone 7 maintain its reputation as being silky smooth.
Interestingly users will be able to disable background agents, and apps will not be able to force them back on again, giving an unusual amount of control to the user. On Android there are task killers designed to repeatedly kill background apps which force themselves on again, showing a distinct difference in philosophy.