We just bought a house built in '68. It has a boiler that feeds a loop that goes around the inside perimeter of the basement. All the baseboards and radiators tee off of this main line, but don't interrupt it. It's not pressurized. My questions are: Is this efficient? Is this normal practice?

What you describe sounds like what's called a "monoflow tee" configuration. See if this article describes what you're looking at:

http://www.pmengineer.com/Articles/Feature_Article/2005/06/03/A-Simulation-Model-for-Diverter-Tee-Systems

This approach has been around awhile and it works. There are some pros & cons too it, but it gets into a the arcane issue of hydronic design. If it heat's the space for you it's fine.

If the basement walls aren't insulated, putting the radiators up against the uninsulated wall is pretty inefficient though. Whether it's worth changing depends on what you intend to use the basement for, and whether you're willing to sink some money into building-envelope and heating system efficiency for a basement zone. (I don't heat the basement directly, but it's insulated to R16-R20 with rigid foam, and stays plenty warm for my purposes.)

It is a Monoflo piping system and if designed properly it works great and is as efficient as any other piping system in a residential application. Its based on a diverter t fitting that has an inverted cone shaped baffle that will divert 30 to 50% of flow depending on how it is installed. I assume that the piping is in the basement and the CI baseboard is on the main floor. Usually you will find coin vents on every radiator as it is hard to purge air from this type of system.