The official website makes it pretty clear that line-in will be 100% supported, and I think it'd be nice opportunity to start fleshing out this sub-forum with useful and interesting topics..

How will it actually be handled on the software side?
WAV, MP3, AC3?
Sample Rate, bit-depth?
The N900 had pretty amazing audio quality (maybe the best), is this going to transfer to it's recording capabilities?

This is a genuinely awesome and surprisingly applicable thing for me, it is so difficult to find something that could do this for me so getting these tech specs is a big deal for me..

Thank you from me, and every other garage audiophile for including such an obscure feature!

Well, we yet have to decide on implementation details. The requirement specs are along HiFi parameters, which means at level of CD sampling rate and format and S/N. This might not apply for mic input on same interface, which needs a preamp/boost that may introduce some higher noise floor compared to line-in level.
Simply go by the purpuse why we got this: it shall turn the device into a viable replacement for audio stereo recorders designed for high quality stereo recording of live events. Yet to evaluate if we can integrate high quality stereo microphones or we will just not bother to come up with stereo mic embedded and rely to plug-in mics.
/j

From (subjective) perspective of notorious bootlegs creator - don't bother with trying to do stereo mics *inside* device (and mod case to actually have some sane distance between mics for stereo recording).

Someone who want to record live events in stereo probably aims for higher quality possible, which means connecting reasonably sized stereo microphone(s) (as compared to internal ones) on cable. In other cases - where quality is important, but doesn't need to be absolute pinnacle (so-called "dictaphone" recordings) good mono embedded mic will do same good as pair of stereo ones, if not better. Not to mention lower file sizes, too.

Not sure if I got it right, so let me ask using an example. In case of my notebook, I can use same mic input in "line-in" or in "mic" mode - the latter have preamp/boost enabled (producing "louder" signal, but more noise), while the former is more quiet, but totally clean.

Now, in case of Neo900, may it be possible to passthrough/ circumvent amp when connected in "line-in mode" (or how it's going to be called - the one, where device accept input on two channels), or, due to limitations in components/possible design/whatever, amp will be always "present" (alongside it noises) on one of audio inputs?

I'm sure you already considered it, but just for the record - in the later (worse) case, a "damage limiting" would be to set up damn amp on both channels, as even totally "disabled", it would result in recording two paths with different characteristic (one with amp, one without).

/Estel

Yes, that's exactly what I meant. Of course we will make sure that a mic amp wil not introduce noise into line-in signal where that amp is not needed.