Camera manufacturers all have seperate systems, and this must be taken into account when buying into said system.
Some are obviously more established than others, and have better support for their DSLR systems (namely Nikon and Canon). Then there are the other less 'popular' manufacturers who want a slice of the pie. Fair enough, they offer things that can compete with the big guns, to an extent. The main problem is that lenses and accesories can be hard to get hold of for smaller brands, and often there is less choice anyway. There is also a less clear upgrade path, with camera bodies topping out at enthusiast level thus offering less choice in the future. This may well be due to change, but at the moment that's how things are.
Pentax/Sony/Olympus et al have less 'in house' support than Canon or Nikon. Often equivalent lenses and accessories to what the larger brands offer are priced at a premium. Fair enough though, there is third party support from Sigma, Tamron, Tokina and others, which helps.
The 1DMKIII may have teething troubles at least, but no self respecting, sane professional would throw away all their gear to buy something by a less supported brand that they're not used to using. They'd just use a different model, say the 1D MKII N.
To me, and many others, it makes most sense to buy into something already well established.
You can't go wrong buying into any brand, but it's wise to look at what's available and go from there. A 400D is a great choice for a camera, you won't be disappointed
