There are very few true 7.1 sources (some of the Blu-ray and HD-DVD discs include DTS-HD Master Audio or multichannel PCM tracks that are 7.1) - most sources are just 5.1, with a small number of 6.1 DTS tracks. For those few instances of more than 5.1 channels in a source, the same downmixing concept holds true, with the rear surrounds shifted into the side surrounds.

Im using a Toshiba A35...is it allowable to split the SL and SR signal to the two rear channels on my 990 so all 7 channels in my system are used?

Yes, it is - it's been done a fair bit by early adopters of HD-DVD and Blu-ray, actually. You'll want to run a calibration disc of some sort and adjust the surround channel trims in the player, to make sure levels are balanced downstream of the splitter.

Great...thanks for the quick response.