This topic came up on one of the other forums during a correction and I thought it might be useful to post the U.S. rule here.
If anyone has any other rules regarding this, please post them as well.
Here is the passage from the Writer's Guide to Style and Usage, p.43) regarding cannot, can not and can not only:
"Cannot, can not, can not only. Cannot is the preferred form except for the rare instance when a writer wishes to emphasize the not, for example, in juxtaposition to can statements: 'You can run and you can hide, but you can not escape me.' When can not only is used, the trick is to remember that not is working with only as a conjunction; can is an auxiliary that must be parallel with the rest of the statement: 'The restaurant can not only serve a delectable lasagna, but also [can] bake [not bakes] a sinful chocolate cake.' "