Does the nikon D3100 have a inbuilt mic?

I'm thinking about getting the nikon D3100 for video recording but does anyone no if it has a inbuilt mic?

Yes, it does.

Yeppers!

I agree with "Vinegar Taster" and "Ry G", but a suggestion and words of warning…

First, the suggestion:
Link to the manual for the D3100:
http://www.nikonusa.com/...100_EN.pdf
Download and read through it to be sure this camera does and has what you need.
For example, page 2, item 14 shows the location of the built-in mic.
Page 198 shows it is a mono mic (not stereo).

Second, the "warnings".
dSLRs are designed to capture digital still images. Video capture is a "secondary convenience feature". This does not mean dSLRs can't capture good video - they can - but that is not what they were built to do. As such, they should not be treated as a camcorder. For instance, starting on page 45, "Recording a movie" is a good place for you to start after learning about using the camera for still image capture.
Note: "The camera may record lens noise". Page 45.
Note: "Each movie file can be up to 4 GB in size and 10 minutes long;" Page 46.
Note: "Live view may end automatically to prevent damage to the camera… " Page 48 - read the entire section.
Note: Captured video files are in h.264/MPEG-4 video format inside MOV files. Be sure the video editor you are using can deal with them - or you are prepared to transcode the video files before editing. Page 198.

If video sequence captures are short, then you should be OK. If they are long, you will have problems (cool-down takes a LONG time).

Video capture has no stabilization. Do not capture video when the camera is handheld. Always use some sort of steadying device.

These attributes are common to nearly all dSLRs that capture video. If video - and audio - are important, use a camcorder. The D3100 has no mic jack so no way to connect external mics. Use of an external digital audio recorder is recommended. This externally captured audio can be synch'd with the video when editing. For $500, a Canon HF M series consumer camcorder is in your budget range.