I agree; check your contract just in case, but generally you're okay to say "as an employee of Foo, Inc., clients included X, Y and Z", even if you don't get into the technical gritty bits of exactly what you did and how (that sort of stuff you'll probably find is owned by your employer). It's much better for you than not being able to say "I worked for Foo, Inc., where I did stuff."
Like andymoo, there were cases where I cannot say just who the work was for, but usually I'm quite free to cite the client by name.