In reply to Dr Avid:
I use a Mamiya 645 proTL for climbing stuff regularly, I think Ray Wood has the same. It handles very well, much like a big 35mm manual slr, obviously you need to be confident in manual focus & exposure.
Even though 645 is the smallest format above 35mm the quality difference is easy to see, in definition and smoothness of tones.
Scanning is a problem/ expense, I have mine done by a guy in Sheffield with an Imacon Flextight, works out ~£12 a pop. The scans far out resolve the 10mp files from my Nikon D200, (which in turn out resolves 35mm velvia, just) and I prefer the look as far as colour and contrast go. Obviously with a RAW file a similar look is always achievable, but its hard work. A big plus is having the transparency as a reference point.
I'd like to have a go with large format, however I think the speed of set-up and slow shutter speeds would make it a pain for climbing photography.
With larger formats, a big benefit is the reduced depth of field which makes it much easier to draw attention to subjects. A lot of my favourite shots are taken at f2-4 with the standard 80mm lens. It gives a look very difficult to acheive with smaller formats. With wider shots the depth of field can be restrictive though, you usually end up with a shutter speed that requires a tripod and a pause from the climber.
I use Velvia a lot but the bigger area means you can get away with 100 and up films without grain being an issue, especially the newer ones.
As for working on ropes, easiest method is usually a low-stretch rope with a grigri and a handles ascender with a footloop. If you're doing a lot its worth making or buying a workseat. Petzl do very nice ones, but a plank will suffice.
If you want to use a tripod, use the cavers technique of tensioning the tripod centre to a bolt or gear. Can't say I've ever bothered though, just handhold.