Interesting. I assume you mean the maxRequestLength on the httpRuntime element, and in theory it should kick in when using .NET Remoting with IIS, but I'm seeing the same results as you.
There's little documentation on Remoting, and Remoting in IIS, and I can't find anything that explains whether this should work or why it's not. There have been no enhancements to DevForce which would have affected this.