There are also a couple of threads lingering around in the General section on both the t-mobile and skyrocket forums which brush on the topic but don't really go into the details.
I decided to post this here as a MOD.
Yes the Skyrocket will support HSPA+/3g on T-mobiles network. How do I know this? I had one and was using it on t-mobile with no problem.
There are a couple radios you can flash to get HSPA+ on the skyrocket:
T-mobile SGH-T989 KID: http://minus.com/mlupRDTvJ#1
T-mobile SGH-T989 VK1: http://minus.com/mlupRDTvJ#4
Telus KJ3: http://minus.com/mlupRDTvJ#2
Here is the Stock Skyrocket radio: http://minus.com/mlupRDTvJ#3
Depending on where you are located/what bands your operator uses I would assume one of these radios would work better than the other.
On a side note a lot of people have just "claimed" that the skyrocket doesn't pick up hspa+. This is not true, it does but you will have to figure out the right ROM/Kernel/Radio combination. I experienced intermittent connectivity on certain skyrocket kernels while running a T-mobile radio etc. Either way I ALWAYS had the ability to connect to hspa+. Most often I would get speeds of between 3-7mbps. I personally used VK1.
When it comes down to the bottom line though, the skyrocket is essentially the same device as the galaxy s2 but with LTE.
IF this helped please hit the thanks button!