I investigated it a little further and it looks like Qt Creator just doesn't know how to start QEMU properly.
Preferences -> Projects -> Maemo Device Configurations -> Test results in the following:
Could not connect to host: Connection refused
Did you start Qemu?
If I start QEMU from terminal via "mad remote poweron" test passes. It takes multiple QEMU starts to actually start emulator, but that's for a different bug.
As for this bug, it looks like on Mac OS X, Qt Creator 2.1.0 (from Qt SDK RC) just doesn't know how to start QEMU (wrong path somewhere?).
In the past I was able to debug Maemo builds on QEMU using Qt SDK Technical Preview.
With Qt SDK Beta I wasn't able to do it anymore. I was hoping that it will be fixed in Release Candidate, but it still doesn't work.
There is a possibility that some remainings of Techical Preview's QEMU are the source of a problem, but it's unlikely - I always tried to remove previous SDK version fully before installing a new one.