On 29 Novemeber the server at ftp.proftpd.org which is hosting the official ProFTPD download packages and source for all other mirrors was compromised and a backdoor has been planted into the source. This seems to be the case only for the 1.3.3c version of ProFTPD.
Original quote from protfpd.org:
[01/Dec/2010]The ProFTPD Project team is sorry to announce that the Project's main FTP server, as well as all of the mirror servers, have carried compromised versions of the ProFTPD 1.3.3c source code, from the November 28 2010 to December 2 2010. All users who run versions of ProFTPD which have been downloaded and compiled in this time window are strongly advised to check their systems for security compromises and install unmodified versions of ProFTPD.
To verify the integrity of your source files, use the PGP signatures which can be found here as well as on the FTP servers.
The source code in CVS was not affected.
Thus, if you grabbed 1.3.3c lately and set it up on your system you'd better act upon it. More info on the subject and pointers on what too look for can be find here.