Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

I use Eagle and can upload the file directly to OSHPark.  Their online upload function will show you what the board will look like.  You can also upload gerber files.  They had a CAM processor script for Eagle that will allow you to generate gerber files if you wish.  Other board suppliers have Eagle CAM processor scripts that will generate the gerber files that they need.
One thing to always do is use a gerber viewer to verify what you send to the board house will do exactly what you want.  I caught a mistake I had made in a set of gerber files I sent in and luckily caught in time and was able to send in corrected files.
There are several free or online gerber viewers available.
DuWayne