I recorded last night with my 5G iPod video, using a line-input dock made by Soap.
Using a Sony ECM-907, the recorded signal is present, but barely audible above the noise floor. As a point of reference, what I was recording is a show that I play seven times a week. I had previously recorded the show with an iriver H120, using the same microphone, same mic placement, etc. In that recording, AGC safety mode limited the overall gain to about 28dB.
I also tried using my Total BitHead headphone amp to boost the signal. That gave me a little more signal, but also more noise.
I haven't yet had a chance to try recording from a line-level source.
Recording was unstable generally. Rockbox would almost invariably eventually freeze during the recording. I wasn't able to detect any pattern about what caused the freezes.