Would it make a difference if the chipset being used was already used for a cellphone that was FCC certified? If I put DD-WRT on my router, do I need to re-apply for FCC certification? (Wondering)
For Wifi it's surprisingly strict:
• Every antenna+transmitter configuration has to be certified separately (that's why Lenovo and other laptop vendors have Wifi card whitelists and refuse booting with uncertified chips installed).
• The software that directly drives the hardware must be certified to conform to the transmission power limits etc.
For DD-WRT and others neither is a problem, because the hardware combination has been certified by the router vendor, and DD-WRT uses the wifi chip vendor's firmware blob to drive the hardware, which is certified by the vendor.
At least for many Atheros-based chipsets they use ath9k instead of the vendor blobs.
Are you sure about that? The fact that not every vendor has such a lock suggests to me that there is no legal requirement for it.