Bundling fonts with an app is not covered by standard EULAs. The font is supposed to be installed on 1 to 5 computers and not on possibly millions of devices of the users who use your app.

FSI has just started to license fonts for iOS apps: http://www.mobilefontfonts.com/
The pricing is pretty fair, too!

Seems like such a weird restriction since I'm using the free version of Museo Slab. I wonder if these licenses are outdated will change any time soon? My understanding is the font is embedded within the iPhone app, and not installed like it would be with desktop software distribution. But oh well... Thanks for the mobilefontfont link!

It's not about what you call “embedding”. It's about whether or not the font software is used. If you embed glyphs in a PDF or just create a logo as a bitmap, the ones using these files don't use the original font software.
But if you embed the font software in an app, you redistribute the software to an unlimited number of people and it is used as it would be installed on the device. That's something very different.

Also, you can use LIBRE fonts from the Google Webfont Directory.
Legally and Free of Charge.
http://www.google.com/webfonts

I just came through this discussion and I wanted to add my two cents as I thought the point of the Mobile FontFonts website was more legibility than licensing issues.

Let say I want to buy FF Yoga Mobile (from Xavier Dupré) on Mobile FontFonts. When take a look at my cart I've got:

FF Yoga Sans Mobile Pro € 159.00
Licenses: 5

At this step I see no difference on the licencing point as I always have to choose the number of devices (1000 max. which isn't realistic in the case of an app)
I thought the only difference concerned legibility issues more than the licencing issue because as you could see on the screenshot bellow, the price of the mobile font is the same than the classical version:

So, from my side, the licencing issue raised by @jujube is not really solved.
I thought the price of Mobile FontFonts fonts would be higher than the classical versions and that I wouldn't have to choose the number of devices (as we don't have this data during the design process and the app would be used by a potentialy high number of users).

With the informations I have, I guess the better solution is to ask the foundry or the type designer himself to know wether the font could be used within iPhone app or not ?

So, what you think ?
Maybe I missed something ?

There is simply no device fee with Mobile FontFonts. You just pay the 159,– Euro to license the fonts for iOS devices. That's it.