I am going to be building my own UPS since i enjoy doing that kind of thing and consider it a enjoyable break from the work world. Here are some URLS i've found on this
http://www.iamnotageek.com/a/115-p1.php <-- Good place to get the basic concept of it all
http://home.netcom.com/~larry_l/battery_ups.htm <-- To see how it is really done. I am going to be doing something like what can be found at this URL.
I'm going to drop a 3000W inverter on something like that setup running with DEEP CYCLE batteries