You could run your UGC website active-active akamai (ie. servers in both colos are serving traffic load) - Lets say the colos are: west coast and east coast. The problem is that if you site has user generated content(UGC) - some users (1%) will find themselves being thown from one set of servers to the others in the middle of their sessions. Given that the east coast database may contain something that they just added and that full bi-directional database replication is hard to do and is delayed - many UGC sites simply run active-passive.

Chris: Flipping is rare.
Chris: Around 1 %.
Chris: But it can flip anytime.
Paul Birnie: but over what time period
Chris: It will flip when an ISP makes a network change.
Chris: This will happen every day, several times somewhere.
Chris: There will be a constant number of networks flipping all the time.