If I substitute my custom silkscreen gerber file and then that off instead if the Itead script generated one, should that be OK?
Yes; they silkscreen whatever you send them a Gerber for. Just make sure the Gerber is what you actually want silkscreened :-)
Several people here have been touting the Iteadstudio service, but unreliable delivery is
really really really bad news.
I don't hear unreliable delivery. I hear ITead sending him exactly the boards he sent them files for. He used a script to generate those files without understanding the inputs to that script. Easy mistake to make the first time, and easily corrected once it's hurt you once.
Personally, I edit the CAM file to include the layers I want for the silk screen. I like to have component values included, for example. Works fine for me. The CAM processor files are really quite easy to work with in Eagle -- as long as you know that that's what you're doing (and need to do.)
That's defending ITead -- they are affordable, and I find them easy to work with. However, if you don't pay the $28 for DHL, shipping is slow, and the boards, while functional, and not top of the line quality. If you want high-quality boards, still at a hobbyist-friendly price, use http://oshpark.com/