AE is set up dual band (plus Guest access). 2.4GHz is channel 1 and 5GHz is set to automatic (which selects channel 149, but, as noted, changing the 5 GHz channel didn't help). AE software is up-to-date.
I'm actually not jumping up and down screaming about this, because for me it's not a huge problem. The iPad works fine on the 2.4 GHz band, and I've seen lots of other threads reporting this problem with the 4.2.1 firmware, so I assume Apple's aware of the problem and will eventually have a fix. I only posted because the OP asked if there were other folks in his same boat.
The thing that I am a wee bit ticked about is that I get better throughput with Guest access than with non-guest. Download speeds with Guest are about 1.5 Mb/s instead of 1.0 Mb/s with non-guest. I assume the encryption is handled in hardware rather than by the iPad's A4, so that doesn't explain the difference. Not a huge deal but annoying; since I want to use AirPlay and (eventually) AirPrint, I'm stuck with the slower access speed. But I figure this is probably a problem with the AirPort Extreme rather than the iPad. And it's not so annoying that I won't just tolerate it until Apple fixes the 5 GHz problem.