Currently I have the +12 agi enchant on my tanking boots. Is the speed boost (8%) from the Cat's Swiftness (http://www.wowhead.com/?spell=34007) enchant is worth the loss of 6 agi?

the mats seem a little steep !

does this speed increase stack with anything else ?

ie will you get the full benefit from it depending upon anything else u have for speed increases ?

does this enchant provide a speed increase in all forms ? and on mounts ?

it does not stack with other enchants or abilities.
a feral will run 15% (not 23%)faster with the skilled talent in cat, and run 8% faster in caster form due to the enchant.

I had just read somewhere (not sure where) that it is good enchant for tanking (or the +sta version (http://www.wowhead.com/?spell=34008)). The extra mobility helps.

I don't want it for kitty so the fact that it does/does not stack with other speed boosts isn't important. FWIW, I think it does stack with some other enchants, at least that is what this guy (http://forgoodnotevil.com/druid/speed.html) says.

I've always thought of Boar's Speed as a PVP enchant. Do you really think the run speed is going to make a difference for tanking? I'd much rather have three more stamina, personally.

The way I look at it is: if I'm far away and Feral Charge is down, how much of a difference is that little bit of run speed going to make?

It's very good for tanking. It absolutely makes a difference in any fight where mobility is an issue. For example, with boar's speed you can get up to Al'ar in phase 1 without a single flame buffet triggering. It makes a difference when closing in on mobs without charge, it makes a difference when being nailed across the room by Thaladred or Al'ar's adds, or trying to get to one of Vashj's lackeys. Think about all the fights where you're running around.

I wouldn't recommend the agility for bears, but boar's speed is only 3 stam, and it comes in handy all the time.