If the .NET is developed and a popular site, then the .COM will likely receive fallover traffic as a result, but the existence of a .NET doesn't automatically make the .COM valuable. If it's undeveloped, and the name is not otherwise special, than take a pass.
A lot of times the .COM was originally owned by the same owner as the .NET and dropped on purpose, and he intends to drop the .NET as well when its expiration date comes up. (does that make sense? :p)