I have two sites that now produce this error. A reinstall of 3.1 kills the errors for a few hours but then, it's back.
It's only when accessing the 'other way' from how the WP site is set up.
http://www.recipebuzz.net/ produces the error but
http://recipebuzz.net/ looks fine.
http://anotherfinemeal.com/ produces the error, but
http://www.anotherfinemeal.com/ loads fine.
The full error is nearly identical for both;
Warning: Cannot modify header information - headers already sent by (output started at /homepages/18/d94445343/htdocs/meal/index.php(1) : eval()'d code:37) in /homepages/18/d94445343/htdocs/meal/wp-includes/pluggable.php on line 897
I tried deactivating all plugins as a test, but that didn't help.
I updated to 3.1 as these two sites appear to have been hacked, but in hindsight, it may have been connected to this.
I'll leave it showing for now as links to the sites should still function correctly.