January 2nd (two weeks before the freeze) will be the last day on which packages that are "new" will be accepted for potato. Packages uploaded after that date will be held for woody.
This will be done to ensure that new packages aren't inserted right before the freeze, and to separate the pre-freeze rush from the actual freeze. (Both are a lot of work for the same set of people). New versions of existing packages will be processed normally, up to the actual freeze, which will happen on the weekend of January 15th. (The only thing that can delay the freeze is if the archive doesn't get the extra disk space it needs.) Package splits, and multi-binary packages sprouting new binaries, are in a gray area and will probably require careful thought. In related news, I'm hosting a Grand Bug Bash Festival on the weekend before the freeze (January 8-9). This will be just like a bugsquash party, but it will last longer and have a cooler name :-) (I note that bug, bash, and festival are all Debian packages... so someone needs to package "grand"). Richard Braakman