Apparently it's a possibility. While there has been no official word that this is the case, there have been many comments made by cast and crew of the series to make fans worried.
Warehouse 13 returns this week, and having seen the first episode already I can say that this season looks to be quite exciting. I'm especially liking the new character additions to the show. But could this be the end of the show? Brent Spiner seems to think it might. He linked to a fan-created site dedicated to making sure that Warehouse 13 does not end after this season.
The Renew Warehouse 13 site details the various concerns that have been raised by cast and crew, and has a survey for fans to fill out. The basic premise is that the show's Nielson ratings are too low, but since the Nielson survey system is hopelessly outdated it's necessary to find an alternate means of showing TV execs just how many people are actually watching the program. It's like a petition... but with facts! Anyways... go fill out the survey. Watch the show when it airs. Hope for the best.