Which DSLR camera has the longest recording time in video mode?

I was gonna buy the Nikon DSLR 3000 but found out it only records for 5 minutes at a time

There are all kinds of time issues and actually 5 minutes is pretty long. Many only go to 30-seconds.
The reason is all that data passing through the sensor and other parts creates a lot of heat that can damage or even destroy critical parts like your sensor.

For that reason you'll just have to live with how ever the camera is equipped.

The D3000 has NO video mode. The D3100 has the video and it can shoot for up to 10 minutes at a time.

First I have to ask if you are going to attempt to use your new dSLR as a surveillance camera?

That is not the reason Nikon or any other company included video with their new cameras.

Second I have to ask if you have had any classes in film/video making?

When shooting video, most "takes" are under 2 minutes and shot using a shooting script. Just look at your favorite TV drama and time the scenes during the program. I doubt that you will seen any scene longer than a full 2 minutes.

Did you know that cameras with CMOS sensors actually get hot after taking long takes? The five minute limit has more to do with protecting the sensor than forcing you to use a shooting script.

Look into how videos or motion pictures are shot and you will see that 5 minutes is plenty of scene run time.

Look at the Nikon D3100. While it can shoot takes as long as 10 minutes (remember the higher the resolution, the more space the video will take on the card) your limiting factors will be the size of your memory card as well as the heat of the sensor as you take your shots

Take a look at your shooting technique and I'm sure you will find a camera that suites your style

Nikon D3100 has the video and it can shoot for up to 10 minutes at a time. You might consider Canon EOS Rebel T2i 18 MP CMOS APS-C Digital SLR Camera
- 18 Megapixel APS-C CMOS sensor
- DIGIC 4 processor with ISO 100-6400 (Expansion to 12800)
- Continuous shooting at 3.7fps
- Full HD movie recording with manual control and selectable frame rates

I'd just like to add that the real reason for this limitation is EU custom regulations. If it can record continous video for too long it's classified as a camcorder which comes with higher import taxes. Heat used to be a problem, but nowadays it's really not anymore.