A couple quick things.
Besides the BIGip take a look at the Alteon AD3, nice piece of machinery. (These load balancer companies have been going out of business lately so make sure you are working with a solid company. That goes for co-location places as well.)
Factor in the cost of setup and support by the people you co-locate with, they usually have the fastest response times. We are looking at Above.net to manage our networks (since the machines are there now), but we quickly discovered that we have to pay for them to 'setup' the network machines according to their standards, this means we get charged from a couple hundred dollars per network device to over 4,000 for some of the more advanced devices (redundant load balancer and firewall) to be setup, I think for the one install we have that was over 50k right there and different colocation facilities deal with different hardware and different setups.
We had talked to a company (forget their name) but they were using a device called NetArcs (could be wrong on the spelling) to do active network monitoring. I think it was just a set of software that monitored the network and the machines and would 'activly adapt' to problems presented by attackers and malicous code.
I wonder how hard it will be to get two colocation companies to deal with each other on setting up the nessesary infrastructure for redundancy. It is hard enough at times to get that done within one company.
