(EDIT: I tried the linux strings utility. I only see the original block's message, and my own's message. Guess no one else has done this.)
(EDIT2: I tried the strings utility on Bitcoin's blockchain. Doesn't look like stuffing information into a blockchain is that unique of a prank, drat.)
Hahaha this is great, there's a couple there now, maybe a couple people picked up on your advice:
Mined by XertroV
the dopefish lives
SNAPE KILLS DUMBLEDORE
What happened?
I put messages inside two of the blocks I mined. Sunny King's primecoin genesis block and a block by XertoV also had messages in them. You can see them in the blockchain by running "strings -n 16 ~/.primecoin/blocks/blk00000.dat" on a Linux machine for example. (You also get a few lines of gibberish, which I'm not sure if they are intentional.) I don't think there currently exists an easy way to inspect the messages further, such as to find what block they're in.