I'm not sure how you can pre-announce something in a way that cannot be later faked.
The best I can imagine is you write it in a text file and post the size / hash of the file.
steve@skx:~$ ls -l 10-march-2009
-rw-r--r-- 1 steve users 234 Jan 12 21:40 10-march-2009
steve@skx:~$ sha1sum 10-march-2009
steve@skx:~$ md5sum 10-march-2009
I don't need anybody to keep me honest, but I welcome interesting suggestions on more neat ways to pre-confirm you have content that hasn't been changed between being written and being released...?
I guess you could use GPG and a disposible key-pair, and then post the secret key afterward, but that feels kinda wrong too.
Update of course you could post the detached signature. D'oh.
Shamir's Secret Sharing could be another option - posting just enough pieces of the secret to make recovery possible with the addition of one piece that was witheld until the later date. Jake wrote a nice introduction to secret sharing a couple of years ago.