I have been looking for similar for ages. It just doesn't exist yet. It's weird that 2 years later nothing can beat the n900 yet. It'd be like a downgrade if something didn't have a a qwerty and multi tasking.

Motorola droid 3 looks like it might fit the bill. BUT it's only in the USA at the moment
SonyEricsson xperia pro looked good a couple months back, but it keeps getting put back and still not released.
HP Pre 3 looked good but webos has died now and reports are the phone is actually a bit of a disaster to use, which is a shame.

Manufacturers just don't seem to be making many qwerty's, and also putting decent hardware in there. The Z is just rubbish imo. Wasn't even good spec when it came out and it is just about obsolete now. I know loads of people who would want 'hardware buttons' but are having to go for phones with only about 2 buttons that rely on touch screen stuff because that is all that is available..
Manufacturers and phone shops look at the sales figures and think 'no one is buying qwderty phones'. Well DUH, if there are none in the shop, people can't buy them. It's a bit of a self perpetuating myth that people only want phones with no buttons.