For what inputs are you using Upsampling. I have set my CD and Tuner inputs to Upsample, but I'm not sure whether to use this setting for DVDs or Cable TV.

What do you think?

I'm pretty sure you won't hear any difference.
Dunno about your Cable TV, but many recent DVD players employ 24/192 upsampling anyway, so there won't be any effect.
That said, I'm not a big fan of upsampling:
only the very best CD players succeed in avoiding sharp treble edges and shifty staging.
I have a cheap 16 bits 44 kHz DAC that sounds just fine.
But I didn't hear any ill effects when setting upsampling to ON in my 990, so the chip must be a benign variety...

I use upsample for my CD input. Upsample can be overlayed on top of any source (PCM, Dolby Digital, or I think even DTS), but at that point you cannot apply additional processing such as Pro Logic IIx. Since I use PLIIx for cable TV and DVD's, I don't use upsample for those.

If yous set pusampling to dolby digital, does if first downmix to 2 chanel, or does it upsample all the chanels?

i've tried upsampling while listening to some CD's. and i didn't notice any difference in sound quality.

i usually use bypass anyway..

If you select upsampling while it is getting a Dolby Digital input, it upsamples the entire 5.1 signal. I'm not sure whether the upsampling comes before or after the decoding, but it does not downmix to two channels.

I've used the upsample overlayed on DD 5.1 and noticed a bit more clarity, which helps on DISH HD broadcasts, and some DVD's. Nice touch== it's not documented in the manual.

How does it work? I only get two channels from upsampling.

I've only used the Upsampling on CDs.
I wasn't aware that it could be used with DD and DTS.

You can use upsample with DD 5.1 and I believe DTS as well, and you'll still get all 5.1 channels. The catch is that there's not enough CPU horsepower left to overlay additional processing modes, so if you have a 7.1 speaker setup you can't use Upsample and Pro Logic IIx together with a Dolby 5.1 source. As sportsound says, it's a slick little undocumented feature - and one that I'd probably use a lot with a 5.1 setup.