I'm getting this, which is kak expensive (although not really from the perspective of non-mobile stuff):
http://www.sonomawireworks.com/guitarjack/
The reason why I'm not going with a cheaper input through the headphone jack:
This is a high-fidelity input. By using the 30pin jack you get top-quality sound (practically noise-free even with the gain maxxed.). The headphone jack inputs are very cheap and convenient, but they create a kind of "feedback" - even the best of them - at higher gain, and produce significant amounts of noise that limits your recordings to song-memo or rough demo level.
Using this, I don't need a separate microphone. Tascam makes a stereo condensor mic for ipad/iphone/ipod. That would be very convenient for me, but it has no separate line in - so I couldn't use other mics. This I can use with my Sm57 or any other dynamic mic until I have the bucks to buy a preamp or a battery-powered condensor. So I'm saving on buying that mic unit by getting this, while losing some convenience.
All in all, this is the best input currently available. Its versatility puts its cost into perspective ($150 retail).
Otherwise, there's the units that forum member Sharonzazz makes - which minimise the noise of a headphone input (pm him for details) or the Tascam link which has a handy XLR mic input which even provides phantom power. You sacrifice fidelity for cost though. Depends what you want it for.
