I also have the disappearing dialog box when uploading or attaching files to email, printing, and downloading. My experience with upload/attach is the same as above. When printing, it just vanishes and nothing happens. However, when downloading, I get the dialog box to browse for a location to save and if it disappears before I select what I want, it downloads to whatever folder was selected when it disappeared. I just updated to 3.6.10 to see if the issue was resolved and it was not.

After upgrading to Snow Leopard I am no longer experiencing this problem, however a coworker running 3.6.12 on Leopard is still experiencing this issue. It would be really nice to get this sorted out, it's quite the headache when you use tools everyday that rely on uploading files.