Mick,
I shoot a lot of gymnastics. You need 1/250 to 1/800 sec shutter speed depending on the level of your gymnast and the event. Floor and vault need the fastest shutter speeds. 1/250 can freeze a split leap or straddle on the beam if you catch the action at the peak, when the action has essentially stopped for a split second. 1/500 on the floor will not freeze the hand and foot movement of top level gymnasts. 1/400 or 1/500 is necesary to freeze the foot movement of even a beginning level gymnast doing a quick walk-over on the floor.
Of course, freezing the action is not always necesary, since motion blur can convey movement. However I sell photos to parents, and most of them have cameras and can get photos with motion blur, so that are always looking for crystal clear shots free of blur.
I always shoot in manual mode, wide open. I use ISO 1600 or 3200 (and now 6400 with the Mark III) to get the shutter speeds I need. The 135 f/2.0 is a superb lens for your application, although I use a 70-200 F/2.8 whenver the light allows. The lower you can get (sit on the floor) the higher leaps, tucks, vaults, etc. will look. If you can get close to the beam and shoot with a wider angle lens from low, the leaps and jumps will look even bigger. The downside is that you usually get the ceiling lights with this angle which can be distracting. I have shot beam with my 200mm f/1.8L + 1.4X (280mm f/2.5) to minimize a distracting background.
Before I started shooting professionally (which gives me access to the floor), I would photography my daughters meets. In most cases I could move around quite a bit to get close to the apparatus she was on.
My shooting/business partner is at the U.S. National Championships this week. Last year he went for the first time and was able to shoot at 1/800 sec at f/2.8 at ISO800. Man it must be nice to shoot in that kind of light!
A nice split leap on the beam from 2004 or 2005 with my first DSLR (a D70):
http://www.pbase.com/d70svrider/image/45445576/original
My daughter on bars last year (Mark II, 70-200 at f/2.8, ISO 1600):
http://www.pbase.com/d70svrider/image/56384723/original
A photo of my daughter with world champion Nastia Liukin at a gym camp I was shooting at a few weeks ago (Mark III, 24-70 f/2.8L using strobes):
http://www.pbase.com/d70svrider/image/83606465/original
Good shooting!
John