I don't know if the problem i run to happened only to me, but i found a solution and i thought i'd post it here in case others have the same problem.
I installed Contact Form 7 on my WP v3.1, instalation was succesfull, even works with other plugins with no conflicts (WP-Mail-SMTP etc.).
The only problem was that in the mails that arrived the message body was empty.
To fix this problem you have to edit the textarea.php located in wp-content -> plugins -> contact-form-7 -> modules.
On line 88 there is an extra white space around the $name variable between the double quotes that you have to delete.
Just delete the extra space and the empty message body issue will be solved.