I just graduated from college with a degree in CS, so I would like to find a job where I can learn more about the field and build up some professional experience.  

I've interviewed at a company that uses their own in-house programming language, and I don't think any others use it.  They have not mentioned using any other languages or what they use for a development environment. 

What should I be concerned by taking a job like this?  If I were to switch jobs later, would I have to start looking for entry level positions again because I haven't gained any language-specific experience?  

Run away, and run away quickly.  Unless you're desperate for a job and are very hungry, this is a situation you want to steer clear of.

I have experience with a company that did this, and the only reason that they did so was so that their employees wouldn't gain meaningful, transferable experience.  It really was all about control.

Others who said here that "programming is programming" are right, but I'd turn that on its head and ask, why not use some standard language for which there is external support, libraries, forums, and a pool of available programmers to choose from?

The only time I think such a situation would be OK would be if the company-only language was for custom hardware.  For example, you have to write everything for the 9000X Gamma-Ray Interferometer using an assembly/machine code specific to that machine.
