Error messages seen in syslog. First two cause no networks to be seen. Also a cracked antenna could cause no networks to be seen, but I haven't seen this.
SDIO Command failed
Responses to SDIO commands not sent from wifi chip. I believe this happens when the baseband is in reset for a critical part of the wifi initialization. blacksn0w appears to make this problem worse, and this is "the wifi problem"
Error initiating scan request: 35
This is a generic scan request fail. If wifi still works(like in third party apps) but only scans don't, this is probably the error. No idea what causes this. Card appears to get a mac address, but wifi doesn't always work.
Grayed out Wifi
This is believed to be caused by hardware. Wifi driver isn't loaded by kernel. Did you drop your iPhone?