If that warning comes with no message ahead of it, it most often means that some included file, such as a module or something, has whitespace after the ?> at the end of the file. Check your contributed modules and custom code to make sure there is no whitespace there.
I've hit this problem when dumping debugging code to the output stream. I think the problem is caused by a call to the drupal_goto() function, which mangles the headers and causes the previous print()/echo() etc to fail. The solution I found and prefer is using drupal_set_message(). This function stores the info I need until the next page load. Another good alternative during debugging is using naïve prints/echos with the devel module's option to interrupt redirects (ie. Stop drupal_goto from acting immediately). This is a helpful option when printing out stack traces or arrays. I find it irritating to have to click through a lot of pages that I'm not debugging, however. Finally, on a more personal note: I know it can be frustrating not to immediately get the support you need. I think you will find however, that people on this list, and in the support forums, are willing to help, and help a lot. As a relative newby I've had my share of unanswered questions, but I find that I can usually get the answer I need by revisiting and rephrasing the issue. Cheers, -Mark