It would be nice to have a way to force minecraft to generate the map out to certain dimensions or based on filesize. This would let me generate a large map during server downtime in the areas likely to be explored, so I can both make an image of the map, and so that the server doesn't have to generate areas likely to be explored during peak time.
There is a "/circleload" command in ServerPort. An admin can type /circleload <distance> and and every chunk within that block distance of the admin will be loaded one by one. This causes them to be generated if necessary. It puts a lot of load on the server while running. Also, I haven't tested it in a while, so worth backing up your map first. You could remove Server Port after you have run the command. There are utilities floating around that allow you to clip your map to a circle too.
You can use a jar called Minecraft Land Generator to generate or extend maps by set dimensions during downtime. http://www.minecraftforum.net/viewtopic.php?f=1012&t=76563&p=1299186#p1299186 It works by starting the server over and over, moving the spawn point each time to force the server to load or generate the surrounding land, then moving it back to the original location when its finished. I usually start with 1000 by 2000 to get a nice rectangle map, and extend from there as need be.
