AFAIK,liquid cooling is good for
1) extreme OC
2) small form factor
Apart from that. A good air cooler can work as good as (if not better) then liquid cooling.
In fact, for extreme OC, it's not just liquid cooling, but something like liquid nitrogen. That's for reason why the whole system must be liquid cooled.
Since Mac will not be OC-able. And by considering the Mac Pro suppose to be a work station, which require stability more than extreme performance. It's not suitable to OC anyway. So, reason "1" is not a valid on the coming 7,1.
And as long as Apple willing to give the 7,1 a decent size case which can fit in a good air cooler. Reason "2" is also not applicable.
So, I don't think the 7,1 really need liquid cooling, which generally give more trouble than air cool. And for a known TDP hardware. As long as Apple use a decent air cooler. The CPU should able to perform exactly the same as if liquid cooled.
Also, a decent air cooler usually quieter, cheaper, and easier to maintain / manage.
I know quite a few computer guys, their comments are more or less the same. All liquid cooling system leak at some stage, just when and how. By considering the Mac Pro can run 24/7 for something like 10 years. I don't think liquid cooling is good idea.