You'll have a torrent of water out of the water pump hole. 
But.. no damage done, and you'll have a torrent of water out of the water pump hole anyway, because most of the water in the block won't drain out through the radiator.
That's what i thought. cheers kev. 
well i'm hopefully going to do some tonight. really want to get it so tomorrow is all about the reassembly. 
i can really see myself doing this for a living. what a rewarding job!
Wait until you're driving 200 miles home from a customer after spending 8 hours lying on your back getting covered in $hit and scraping your knuckles and stinking of oil every day for a while..  
Apart from the driving far bit that sounds awesome  
on a serious note james, what would you say are the highs and lows of being a professional spanner twirler?
Well for starters, I am not a professional mechanic by any stretch. I'm an enthusiast, who has become proficient in a number of jobs, and hence offer it as a service in my spare time...
For me, the highs:
 - Sense of achievement when you fix something or work out a problem
 - Pride in my work - I feel really great knowing I've done something properly and not cut corners
 - Reputation - I enjoy the reputation I generally get as a result of the above
 - If solo - all the benefits of being your own boss
 - getting to meet great people, making new friends, being appreciated
 - the money is reasonable
The lows
 - some very early starts, very long days
 - if at a customer site, and only outdoor facilities, it can be miserable in the winter
 - your own car constantly taking a hammering. My MV6 always has a bootload of tools and is thrashed around the UK. Remember you must keep your own car well serviced, even if you can't be bothered - as without it you're stuffed
- Unforseen problems. For example, if something breaks and you don't have a replacement
- IF working away, facilities, EG, jobs under car tricky if only a gravel surface is available
- Although very few and far between, there will always be the odd unreasonable customer