Update: as of 6/13/11 we have changed the 4 byte magic number in weeds to fix some problems and help support other things like bitcoin-abe that we now have a working version for weeds presently found in my patched branch of John Tobey's bitcoin-abe found here: https://github.com/sacarlson/bitcoin-abe  .  to upgrade to the new magic number simply update your bitcoin.conf.weeds file from MultiCoin or MultiCoin-qt github software distribution or the updated link found here: http://exchange.beertokens.info/docs/multicoin/bitcoin.conf.weeds.  I have also found that I have to start MultiCoin from a clean empty wallet -datadir or you will still have problems supporting bitcoin-abe.  If you don't use a local running bitcoin-abe then you don't need to mess with any changes of the -datadir wallets to make corrections, just replace the bitcoin.conf.weeds file you now have and restart multicoin. So if you now fail to connect to the Weedsnet and see PROCESSMESSAGE MESSAGESTART NOT FOUND on the console you will know you have not upgraded your bitcoin.conf.weeds or you are running a version of MultiCoin older than 6/7/11.   example the way I set mine up is: cp bitcoin.conf.weeds /home/sacarlson/.bitcoin/weeds/bitcoin.conf ; bitcoind -datadir=/home/sacarlson/.bitcoin/weeds/ ;  the bitcoin.conf.weeds you us should be the new updated version that you could have got with wget http://exchange.beertokens.info/docs/multicoin/bitcoin.conf.weeds  or from the github sources . The one at my exchange site is the most experimental version that is modified first, When I see that works I also transfer the changes to github.  Oh another thing I forgot to say was that the feature for changing the 4 byte magic number is also a somewhat new feature and you will also need to upgrade to a version of multicoin no older than release date of 6/7/11 of commitish 62c428988ec76bc97fe934c76ad8d77b7060229b or later from: https://github.com/sacarlson/MultiCoin.  As far as I can tell multicoin-qt has always had the changable 4 byte magic number feature from day 1 as it was released after 6/7/11 with the merged features from Multicoin.