I’ve found the memory location of the video recording limit (20 minutes on the D5100) and change it to 1 minute to allow for faster testing.
Here’s a really exciting video where is stops record after 1 minute with no user involvement.
[Update 40 minutes later]
The D3100 10 minute limit has been located, and the D7000 20 minute limit has also been located.
[Update 30th March]
Here’s a screen snap of a 25:59 minute video (1080P 30fps @18mbps) I just recorded on my D5100. At lowest quality settings (424 30fps @4mbps) it says I can record 59minutes of video, so I assume the it doesn’t display hours because 1h 59m @4mbp is 3.5GB which is around the size of my ~26m video.
[Update 1st April]
Patch available from here: Nikon D5100 Video Beta Testers Wanted
[Update 24th April] A web-browser cross platform patch tool is also available.