Outsourcing the data centre - pros and cons

Next year our firm moves to new premises in a building currently being constructed, which means the IT team has a challenging and exciting project ahead.
As we are utilising an integrated fit out, decisions around the data room need to be made within the space of a few weeks.
Inevitably, the issue of outsourcing our data centre has come up.
Historically, we have kept our servers in house and there is a certain level of comfort that we derive from that.
However, we are a law firm - IT is not our central service offering so I am keeping an open mind about whether outsourcing the data centre is for us.
There are two models we are considering, co-location, where we still own the equipment and just house it remotely with a dedicated organisation and true outsourcing where we simply "rent" server space.
I found a conversation on Tech Republic very helpful in this area: http://techrepublic.com.com/5208-11179-0.html?forumID=7&threadID=188756&start=0
An interesting thing to note is that outsourcing/in-sourcing is a cyclical trend with pendulum currently swinging back in the in-source space.
What has become clear to me through researching our options is that outsourcing will only work if your data centre provider is truly a business partner who understands your needs. From this kind of relationship innovation can grow. Many outsourcing projects appear to have failed due the lack of flexibility and agility compared to administrating servers in house.
Another point is that outsourcing may not necessarily save you money in the long run if your current IT team is running the system well and within budget.
However, as city space becomes rarer and rarer and rents continue to rise, companies do need ask themselves if housing servers is the best use of square metres. An off-site dedicated facility will undoubtedly have better cooling, power and security than you can provide in a city high rise. There are number of issues that I have not yet resolved in my mind:
1. Introduction of critical point of failure in your link back to the offsite facility.
2. How to maintain proprietary products that may need maintenance at a server level by third parties.
3. How agile can outsourcing be? Am I prepare to wait several weeks for changes to be made?
4. How will this mind shift affect the partners (the owners of the business) and IT team.
I am not discounting out-sourcing, but at the moment I haven’t fully realised the business case for it.