You can record at 60fps (at up to 720p resolution) if you modify a plist (I dont know if it requires a jailbreakI know I modified some plists before jailbreaking and the effects worked, but this particular mod I only did after jailbreaking so I cant promise itll work without.)
To enable 60fps video recording at up to 720p resolution, locate the file /System/Library/PrivateFrameworks/MediaToolBox/N94/AVCaptureSession.plist and make a backup. Then modify a copy of it as follows. First, identify which video profile your favorite app uses that you want to change, within Item 0 of AVCaptureDevices. For me, I use the ProCamera app which has 4 video quality setting to choose from, and I wanted to change one of them to this.
First I changed all the video profiles in the plist to capture at different sizes (I went with 300, 310, 320, etc. pixels). Then I recorded a video in each mode, and on my computer checked which video was at which resolution. Having thus identified the plist key I wanted to change, I put all of them back at their default values except the one I wanted to change I renamed with original at the end, and renamed a copy of AVCaptureSessionPreset1280x720p60 in its stead. The TemporalNoiseReduction key had value 11, but I had to drop it down to 1 in order to get it to work fast enough for 60 fps. It works at 0 too, but then the video looks wicked noisy.
Once youve recorded 60fps video, you need to change the playback rate to 30fps (or whatever you want). I think that iMovie can do that, but I dont have iMovie so heres what I did.
On a Mac you can use Apples free Atom Inspector (available through developer.apple.com or connect.apple.com) to change the movies timescale from 600 to 300 (or whatever you want playback speed to be) in both the Movie Header and the Media Header for the video track.
That only affects the video track though. For audio I used Ambrosia Softwares WireTap Studio (costs money) to double the length of the sound track without modifying the pitch, then I used Subler (free) on passthrough mode to put the lengthened audio and video tracks back together into one movie.