It's also worth mentioning that multi tenant implementation is *hard*, especially when used with ESL or mod_xml_curl etc.
There are some existing systems that use FS which give you decent multi tenancy support, or with some hard work and a lot of testing you could do it yourself.... It really depends on what you are trying to achieve.
Multi tenancy comes in very handy when you want to allow burstable resources, for example a customer could idle at around 2 CPS, and then burst into the thousands without requiring a machine upgrade. Generally speaking, it allows you to be more flexible, but it really depends if you've done a good enough job getting it configured correctly. If it's not done correctly, you could shoot yourself in the head... cross customer CDR contamination, internal calls diverted to the wrong domain and all sorts of nasty things.
If you're new to FS and want to go with multi tenancy, and need something out the door asap, you'd honestly be better going with one of the existing systems that uses FS. However if you've got a couple of months to burn and a good in-house programmer, it might be worth doing it yourself.
My two cents at least.
Cal