Yeah, it takes 2 slots to ensure the card get a better cooling, bigger surface of cooler(2 slots) can make a card run more cooler than a single slot card...
The single slot 4850s ran pretty hot, which is why many AIBs used bigger coolers. The cooler was intended for lower clock speeds and the 4850 was originally targeted at the 9800GT rather than the 9800GTX. Anyway, some single slot 5750s are popping up and I'm sure that as supply improves AIBs will design some single slot 5770s.
The 4850/4830 reference were single-slot, however few AIBs use the reference cooler. The best probably is a 5670.
Currently there are no single slot HD 5670s. The closet is the ASUS EAH5670/DI/1GD5 Radeon HD 5670 (Redwood):
I checked with a neighbor that does network maintainence and he told me he would check with his China vendor to see if he could find a PCI-e 16x video card that goes in the very top slot and has the cooling fan system going up towards the power supply.