Did you buy a really fast SD card? If not, then the card cannot write as fast as the information is coming in. So the buffer gets filled up and the video has to stop to write the video onto the card. Here is a forum post about the same problem your having, except with a Pentax K7 but should translate over the same. It seems if you want more video recording time then you'll want to look at getting a Sandisk Extreme III or faster.
http://forums.dpreview.com/forums/readflat.asp?forum=1036&message=33695727&q=video+minute&qf=m
I just purchased PENTAX K-x and its a great camera by all means. However I am facing one problem. I cant shoot the video more then a minute. It automatically stops shooting after around 1 minute of shooting. User manual says there is no any such limit except the memory limit. I have 2 GB card installed and have enough memory to shoot longer video. If somebody can guide me who has experience with PENTAX K-x it would be great.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic