There are only two products on the market (that I know of) that provide a STEREO line input for the iPad: The Line 6 Mobile In audio interface, and Sonoma's GuitarJack audio interface. I've owned both, so allow me to compare the two...
The Line6 Mobile In is cheaply constructed in a plastic shell. Even the parts that look like metal in the picture, are just grey plastic. The Mobile In provides two 1/8'' inputs, one for a guitar (it includes an adapter cable) and another for stereo line-in. Only one input will function at a time, I believe (I purchased this purely for the stereo line-in, as I am not a guitar player at all).
The Mobile In DOES NOT WORK for stereo line-in on a new iPad using Garageband or any other audio app I wanted to use it with. Their company claims it's supposed-to, but it doesn't. The stereo input is treated as mono, defeating the entire purpose of why I bought it. Ended up returning the product since I had no faith that their company would be able to resolve the problem in a timely fashion. As I understand it, they still haven't fixed that issue.
The GuitarJack has solid construction and a sleek metal body. You can immediately tell it's a quality device. Its stereo input worked perfectly on the new iPad, whether it was in GarageBand or the other apps I had used. Note, you may have to configure it in one of their recommended apps first, but that's quick and easy to do. From what I understand, you can use both its 1/4'' guitar input and 1/8'' stereo input at the same time, but I haven't bothered to try that yet.
You can use the GuitarJack directly with the iPad, however you may need to remove whatever case you have your iPad in (as I did with my INVELLOP case). When using the GuitarJack this way, you will need to lay the iPad COMPLETELY flat (as in, not tilted, even a little). The other option is to use an extension cable, which you WILL want to get; even if the iPad and GuitarJack are on a perfectly flat surface, it's still too easy to accidentally disconnect the two. Sonoma markets their own extension cable for the GuitarJack, but it's pricey ($25+). Instead, I purchased a cheap, no-name iPad dock extension cable that is over a foot longer than Sonoma's, one-fifth the price, and works just fine with the GuitarJack.
One annoying thing I found was that when the GuitarJack is connected, the iPad's built-in speaker and headphone jack become disabled, and you need to use the GuitarJack's headphone output to hear *anything*. I understand the input jacks taking precedence over the iPad's built-in mic (and I understand that the iPad's built-in headphone jack can also be used to connect mics) but I really wish it didn't auto-disable the speaker, and acted more as an auxiliary headphone output instead. I wanted to be able to mic monitor without NEEDING headphones, but I usually keep headphones with me anyway, so it isn't thaaaat big of a deal.
Minus one star for making an extension cable a basic necessity (and additional purchase, where they *attempt* to rip you off) as well as the disabled speaker/headphone thing. This is an expensive iPad accessory (it costs about twice as much as the Mobile In), but it's constructed well, looks dope, and, most importantly of all, IT ACTUALLY WORKS!!!