I know it's an old thread but it seems to be an ongoing problem, and I think I learned something new. I just burned some iso's yesterday with Brasero and didn't have an issue. Today, I downloaded Ubuntu 12.04 and Brasero wouldn't burn the iso! It started showing it's "checksum" message with progress bar but nothing happened.

What finally fixed it for me was after selecting "burn image", it gives you the source and destination screen. When you select the iso image you want to burn, look at the bottom of the screen; You will see "Image Type" and a drop down box with "auto detect" as default. Click on it and select "iso 9660" (if it is an iso ). That is what worked for me. I know it won't fix all Brasero issues, but I think the autodetect doesn't always work correctly. Hope it helps!