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You can have AP's as dense as you want (the 5ft limit is good) as long as they aren't on the same or overlapping channels. Your question doesn't seem to be really about CSMA/CA, but on how channels can interfere.

I don't know the exact signal that it would take for a device to determine that it was busy, but I will take a guess at around -95dBm which is pretty much the high end of receive sensitivity for most 802.11 devices.

From my experience, it would have to be a signal that is modulated the same as the radio to "stop" it from transmitting. With 802.11g it would have to be DSSS or OFDM, and 802.11a it would have to be OFDM. Someone may need to chime in on that to confirm or deny.

In order to have two AP's (one on channel 1 and the other channel 11) actually hear each other and interfere, they would have to be unusually close to each other. In a lab environment I can have them 5ft apart without any unusually dropped packets or throughput loss.

Let me know if you need more info!

Gene