If any Canon d-slr camera is within $500 of the price of the Nikon D300s, then that Canon becomes a competitor for the D300s, no matter the capture format of the sensor. Many newer buyers do not put much stock in how the big manufacturers attempt to position their products; buyers are many,and varied, and many buyer look at a camera that costs, say, $1,XXX and they then compare other cameras that are priced at $1,XXX.
Canon and NIkon have been trying very hard to compete at subtly different levels. IN my estimation, the Canon 40D and 50D are in no way competitors with the D300...the bodies are very different in capabilities...and yet the marketplace considers the Canons and the Nikon to be competing against one another ,when the Nikon D300 is actually of a slightly different class than the 40D/50D bodies.
I think Canon's EOS 7D moniker, if it proves to be true, is their answer to the D700's feature set/price point/niche of FF with moderate MP count, built-in flash. AND, and this is the prosumer, MWAC hook, on-board video. Canon really does not have a D700 competitor right now, at least in terms of price and feature set and body robustness and AF capability: to make the 7D compete well against the D700 and D300, my feeling is that Canon finally HAS to do something about the AF system's design,performance,and specifications; a lot of consumers look at Canon's enthusiast/semi-pro bodies and see the 9 AF points, plus "hidden" assist points, and the see Nikon's 51 point A system with 3D tracking, color matrix metering,and so on, and the Canon systems' AF kind of loses the specification wars.
As the original Canon 5D, and later the Nikon D3 and D700 have shown, a full frame sensor with only a moderate (12 to 12.8) MP count delivers a mighty good picture...if the 7D happens to be priced within $500, or better yet,within $300 of the D300s, the Canon's feature set will look VERY,very tempting to tens of thousands of enthusiastic and semi-pro and pro shooters. Even if priced HIGHER by $500 than a D300s, the Canon's "7" name might be designed to draw an unstated comparison to the Nikon D700,and it would be an easy thing for Canon to continue to offer a great sensor, wrapped around basically the $350 Elan body-quality that they have been building in for so,so many iterations of mid-level models (10-20-30-4050D and 5D and 5D-II). Canon could easily price the 7D higher than the D300s and just a bit lower than the D700,and have a very successful "competitor" that would steal D300s and D700 sales.