automatic currency updates is something that has been discussed for a while. It really wouldnt be too difficult to do, I could write up a cron job that will run every X minutes/hours to average the mtgox USD rate and update the zencart rate.
I have to finish a few other projects I've been working on in my free time before I can write that script though.
playing with it more and learning as I go, I was able to manually add Bitcoin as a currency in the zen cart 'currencies' table, and set it as the default currency, and added its symbol as 'Ƀ'
the exchange rate is set the the database as well. I could just write a script which gets the mtgox exchange rate every 1-5 mins and run a sql command to update the exchange rate. Is this right?