Is audio line in stereo?

Yes, it is in stereo.

You do not mention which model you have, but the specs on the iSight G5 are listed HERE and they list -

"This analog line input features up to a 16-bit stereo, 44.1 kHz sampling rate.

The 3.5 mm audio input minijack accepts line-level stereo signals. You can also connect a stereo miniplug-to-RCA cable adapter to connect stereo equipment to the computer."

It is stereo but the inputs are not a balanced. They are line level, high impedance inputs suitable for the outputs of consumer audio gear, but not un-amplified microphones or pro gear with balanced outputs. For those sources you will need adaptors, preamps, or alternate input devices that connect via USB or Firewire.

Examples of the latter are M-Audio's MobilePreUSB or the multi-channel audio interfaces made by MOTU.

If you really want the full set of specs, take a look at Apple's Audio Developer Note.

"If you really want the full set of specs, take a look at Apple's Audio Developer Note"

which, interestingly states that the line inputs are capable of 24bit/96kHz recording as opposed to the '16bit/44kHz quote on the 'External ports and connectors' page. The first machines to feature this I believe. Means less of a need I guess for external soundcards etc.

Balanced signals have three wires: signal +, signal -, & ground reference. The signal is "balanced" above & below ground reference, hence the term. Unbalanced signals have only two wires: signal & ground reference. The signal is always "unbalanced" above ground reference.

Usually, but not always, you can convert a balanced signal to an unbalanced one by tying together one of the signal wires (usually signal -) & the ground reference. Your equipment manual will generally tell you when this can't be done; otherwise consult the manufacturer or a good musician's supply store if you are in doubt about the cables or adaptors you will need.

BTW, many mixers have aux outputs that are unbalanced duplicates of the main balanced outs, sometimes marked as recording outs. Again, a good manual will tell you how to hook up the outputs to unbalanced gear.

That will work ... but then you knew that. 🙂

You should now know that since the Mac input is stereo, you could also use a 1/8" stereo mini-plug to two mono 1/4" phone jacks adaptor if you want to use both inputs of the Mac, with another 1/4" phone plug cable for the other input. That could come from any other unbalanced, line level mono source, for instance the second channel on a dual preamp.