It really depends on what format of photograph you are taking. If you shoot Jpeg, the shots are already pre-developed and compressed on camera, any further image edit on your machine will cause a degradation of image quality as each save compresses the image further.
I use a MacBook Pro when out and about on a shoot, but do final processing on my Mac Pro. The MacBook Pro gives me a good idea of what the final image will look like, but the Mac Pro is hooked up to an Eizo CG220 monitor which displays the full Adobe Colour Gamut, so for colour correction prior to print etc I see exactly what the picture will look like on my screen (colour calibrated screen and printer). Hence I do final processing on the Mac Pro.
If you shoot RAW (digital negative in simplified terms) then convert to 8- bitt / 16-bitt etc TIFF files, you need a RAW batch processing program to allow you to do this. Depending on the camera you have (and it's ability to shoot in RAW mode) the camera manufacturers usually produce their own software for RAW editing.
Personally, I use PhaseOne's Capture One Pro software for RAW edit / batch processing. Excellent prog. (recently updated to Capture One 4)
Photoshop CS3 to add additional effects filters etc
I'm giving Aperture 2 a try out for cataloguing and customer viewing etc. It's proving to be ok so far, much improved over the first version. Allows you to change the picture without effecting the original file, so if your customer would prefer a different 'look' to the picture, you can add changes and compare many copies of the same picture. (It's also very good for arranging pictures for albums etc)
That's just me. The progs mentioned above do all I need them to do, but do remember the better the shot you take, less post processing is required (depending on the effect desired for the final shot obviously).
