Configuring Design "Exception" Strings for "Mobile" Devices
There are two different versions which we have used. The first is the same one which @Marius made mention of minus the specific "Fennec" mention (which is no longer in the UA string of FF Mobile). I got it from similar sources:
iPhone|iPod|BlackBerry|Palm|Googlebot-Mobile|Mobile|mobile|mobi|Windows Mobile|Safari Mobile|Android|Opera Mini
The second is the same, but with the term "Mobile" removed so that the mobile version of a site is not served to the iPad, which includes Mobile in it's user agent. Apple even makes a point to call this out in TN2262:
Safari on iPad is capable of delivering a "desktop" web experience, and users will expect this experience since iPad has a large screen and fast network connectivity. If you have a version of your website that is optimized for mobile devices with small screens, do NOT serve this mobile version to iPad users.
...
Note that the Safari on iPad user agent string contains the word "Mobile", but does not contain the word "iPhone". If you are currently serving mobile content to any browser that self-identifies as "Mobile", you should modify your user agent string checks to look for iPad and avoid sending it the wrong version of your site.
Based on what I see on the FF Mobile, using "Android" in this string will cause trouble if you want the desktop theme served to all tablets (and not just iPads) since it's been decided to include it in the UA string for various reasons.
I've honestly not personally tested the coverage of the above strings. I do know this: It's a complicated mess! :)
Note: If you are setting this up on EE 1.12 or earlier (haven't checked 1.13) you will need a patch from Magento Support for the FPC to work properly. It fails to take design exceptions into account in the release build.
Update 1:
The noted bug requiring a patch for design exceptions to work still exists in EE 1.13.0.0 and 1.13.1.0. There is a patch available from Magento support and it's on their known issues list: SUPEE-1598
I have added 2 design exceptions, one for tablets, for which I serve my desktop theme and one for phones, for which I serve my mobile theme. I add the exception targeting the tablets first, and then the exception targeting the mobile phones
Tablets
iPad|Silk|Kindle|Xoom|SCH-I800|Tablet|GT-P10|SC-01C|SHW-M180S|SGH-T849|SCH-I800|SHW-M180L|SPH-P100|SGH-I987|zt180|HTC(.Flyer|\\_Flyer)|Sprint.ATP51|ViewPad7|pandigital(sprnova|nova)|Ideos.S7|Dell.Streak.7|Advent.Vega|A101IT|A70BHT|MID7015|Next2|nook|Windows NT|DROID RAZR
Phones
android.+mobile|avantgo|bada\/|blackberry|blazer|compal|elaine|fennec|hiptop|iemobile|iP(hone|od)|iris|kindle|lge |maemo|meego.+mobile|midp|mmp|netfront|opera m(ob|in)i|palm( os)?|phone|p(ixi|re)\/|plucker|pocket|psp|series(4|6)0|symbian|treo|up\.(browser|link)|vodafone|wap|windows (ce|phone)|xda|xiino|(Android.+Mobile)|NOKIA|SymbianOS|N900|BlackBerry|Mobile
This works pretty much 100% as expected I must give credit, and a huge thanks to beeplogic who's regex I used as my foundation, Thank you beep!
Here is a list of possible values. I'm almost sure that these are not all the possible values, but it covers most cases:
iPhone|iPod|BlackBerry|Palm|Googlebot-Mobile|Mobile|mobile|mobi|Windows Mobile|Safari Mobile|Android|Opera Mini|Fennec
Sources:
http://magebase.com/magento-tutorials/magento-design-exceptions-explained/
http://inchoo.net/ecommerce/magento/magento-mobile-theme-imobile/