There were a number of reports that Advertising Manager was causing an error to WordPress 2.9. Specifically, the error was appearing in all pages (admin and blog), and was something like:
Warning: fread() [function.fread]: Length parameter must be greater than 0 in (wordpress)/wp-includes/pomo/streams.php on line 113;
This is an error in WordPress, and it did not read some language files correctly. I removed the offending language files, and the error is fixed. I will also submit a bug report to WordPress.
In any case, I suggest upgrading, especially if you have Greek, Czech, Swedish, or Portuguese (Portugal) language in WordPress.