Canon Ixus 860, 8MP, tiny sensor
Panasonic G1, 12MP, "4/3" sensor
Canon 500D, 15MP, "APS-C" sensor
Canon 5D-II, 21MP, full frame sensor
All of them had their respective "standard" zoom, set to 28mm equivalent.
All of them set to around F:5.6 or F:8, and 800 ISO, which I find to be representative. Except the IXUS, which I let use its auto-ISO setting which selected 400, wisely since such small sensors suck above that.
Crop from Ixus:
Crop from G1:
Crop from 500D (T1i):
Crop from 5D mark II:
(Big pic when click.)
Update: it's coincicental that the megapixel sizes in my cameras follow the sensor sizes neatly. You might well have a tiny sensor with 14MP and a full frame sensor with 12MP.
At the top is a neat diagram I found which, except for being slightly bigger than real life, perfectly illustrates the four sensor sizes above (two of them even mentioning the same cameras!). (The EP-1 is "4/3" like the G1.) (Don't try to find meaning in monikers like "4/3" and "1/8", they are complete gibberish with no relation to anything real.)
Update: the Canon 5D mark 1 was the king of high-ISO performance for two years. It was impressive that you could get usable pictures at 1600 ISO, albeit rather noisy. No other camera came close for a long time.
But the 5D Mark II kicks it all across the field. At 1600 ISO I don't see any noise at all! In fact I basically can't see any quality difference going from 800 to 1600. And they did that at the same time as almost doubling the resolution! That is f***ing impressive.
Update: even more so 6400 ISO, which I had kind of dismissed, but shouldn't have. At default setting, it's good. But it gets even better if you turn off high-ISO noise suppression (custom settings) and handle noise in the computer. (And this is from JPG, I haven't tried RAW yet.)
I thought only Nikon D3 could do this, and the D3 has only nigh half the pixel count.
Frankly, this goes beyond my expectations. When they boosted the resolution that much, I was sure they'd blown the chance for really good high-ISO performance.
Note: for a long while, I thought that noise was the only penalty for using the high ISO settings. But there are other issues too. Most importantly, you loose sharpness/resolution/detail, and that can't be recovered. The dynamic range also suffers some, and colors may lose some clarity, though with the new generations it seems to be minimal.