An attempt at a quick summary of the systems:
Canon - biggest but not best value. Will be around for ever, more lenses than you could fit in your house and they're easy to get hold of. Biggest for a reason - solid choice. USM in just about every lens. Range - budget to pro.
Nikon - a smidgen behind Canon in size, should be around for ever, more lenses than you could fit in your garage and also easy to get hold of. USM in just about every lens. 2nd biggest for a reason - another solid choice. Range - budget to pro.
Sony/Minolta - based upon Minolta who were 3rd biggest in 2005. Every lens you'll ever need, and then some, but be prepared to have to hunt 2nd hand or import for some of the more exotic stuff, or for reasonable prices on certain lenses. Distinct lack of USM focusing makes focus less refined, and budget bodies can therefore struggle with their big lenses - don't try to mount a 600mm to an A100! Wireless flash and stabilisation built into body their wild-cards. Range - budget to semi pro - pro body announced and expected 2008.
Pentax/Samsung - very solid lineup of bodies with weird and wonderful primes. Stabilisation built into bodies a huge bonus. You'll never struggle for lenses as long as you don't mind manual focusing, they take every lens ever made for Pentax. AF selection pretty limited, be prepared to import and scrounge ebay. Range - budget to semi-pro - no pro body announced?
Olympus/Panasonic/Leica - weird and wonderful mount brings advantages + disadvantages. Killer compact telephoto options, f2 zooms and awesome pro-glass, but small sensor leads to worse high ISO performance than other brands, including highlight clipping earlier up the scale. Image stabilisation in body, distinct lack of primes. Tiny 2nd hand market - again be prepared to import and scrounge ebay for a fair price. Range - budget to pro.
Sigma - total oddball, only one model but it uses a weird and wonderful sensor which, depending on who you believe, is either stunning or below par. Decent lens lineup as Sigma make sure their own users can get hold of Sigma glass. Almost no stabilised glass available though. Range - one model only, mid-range.
Enjoy shopping