Don't panic, the same thing happened to me and coutless others. Usually this happens because you have been using your Mac for a while before attempting to run Boot Camp Assistant. Over time, small issues with the HDD come about that prevent it to be installed (Boot Camp needs a hard drive that is in very good condition to do its job).
Anyway, all this means is you will need to run a scan and repair these errors using Disk Utility.
but here's the catch -- you cannot repair errors on a HDD that you are currently using OS X on (ie. your internal HD). So, to fix the errors you will need to boot to your Mac OS X install disk and do it from there. So here are the steps:
1. Insert your Mac OS X install disk and double-click the "install Mac OS X" icon, then restart you mac as though you are going to re-install the OS (you won't get that far though -- we won't actually be installing anything). Alternatively, you can insert your install disk, then restart and press the "C" button at startup, but the first way is easier if you're already in OS X.
2. Eventually you will come to the Mac OS X installer screen. First choose a language and click the arrow. But STOP before you get to the screen where you choose which HDD to install mac OS X to. You dont want to do that. Instead, you should see a menu bar appear at the top of the screen like you normally do. Go into the "Tools" menu and choose "Disk Utility" (at least I think its the tools menu. I can't remember off the top of my head, but if you look through all the menu you will find Disk Utility in there somewhere).
3. Once disk utility opens, select the HDD you wish to repair from the list down the left of the window. Then click the "First Aid" tab in the main area (probably selected by default), then click the "repair disk" button at the lower right. This will scan your HDD for errors and correct them.
Now, if there are no serious errors with your HDD (which there shouldn't be if it is a new Mac) then it will repair the disk and display green writing at the end telling you this is so.
4. Quit the installer and restart the Mac into Mac OS X as normal.
5. Use the Boot Camp Assistant again -- everything should run smooth.
6. If you still can't partition your drive ,go to
www.macosx.com/forums and post the message "Thank the Cheese is a liar -- someone help me!"