If you ask for things like code, requirements document, etc, they'll probably send you the best of what they have and that might not be the status quo. At the same time it doesn't hurt to take a look over such things and see how they handle the request.
Since 90% of the iphone shops have a life of about 2 years (most people have jumped on the iphone bandwagon in the last few years) I wouldn't hold that against them but I'd make sure they have a development background and didn't start their development career on the iphone in the last 2 years [1]. If I was outsourcing any type of work (iphone, web, desktop), I'd want to work with a group that has been through a handful of the ups & down's of developing, delivering and supporting software, plus has the client skills to match.
Meaning they can communicate, know how to manage and know how to run the development side as well. I'd like it if they at least had some development history/experience in C or C++.
Also, do they have artists and such in house or do they outsource the asset creation? (Maybe its not needed for your app besides an icon & splash screen).
What software do they use for bug tracking? How do they manage their development cycles? Do they use a methodology? (waterfall, agile, etc)
Do they offer support? How much is it? Contract? Per instance?
Do you get the source? You should, you paid for it.
Speaking to them is very important, and your gut will tell you much about them. If you can drop by and checkout their shop that's cool too. But don't hold a small shop against them - it should just mean better rates for you due to lower overhead.
If they've done consulting work, and have shipped app's that's a benefit. Specifically question them about ad-hoc distro.
And importantly, have they worked with the technologies you need (say openGL for a game, or consuming web services for a network related app)? Again, not necessarily a strike against them if their smart and eager, but you'll be appropriately aware of their current abilities.
Good luck!
Also, if they're willing to work on equity alone, I'd be worried. Developers are making good money on the iphone and I see no reason to take equity only. I consult on the iphone, and I won't accept equity deals. Cash is king.
[1] What I mean is that, I would expect them to have developed on other platforms (web, desktop, other mobiles) NOT just the iphone. So if they started programming in the last year, on the iphone alone, that's probably not a firm I want to work with. If they had developed desktop applications for the last 5 years and then came to the iphone world in the last year or so, that's cool. I just want to work with people that have gone through the first couple years of development - those are great great great learning years (but don't want them doing it on my dime).