It turns out that when you drag and drop a file between two Finder windows on the same Mac OS X filesystem you end up moving the file. For me that recently begged the question "How do I copy files when using drag and drop?"

Fortunately the answer is simple and reasonably easy to remember: When working with Mac OS X, to copy a file (or multiple files) from one Finder folder to another, just hold down the [Option] key when dragging your files from one folder to another. This magic keystroke tells the Mac Finder that you want to perform a copy operation, and not a move.

Fortunately (on my MacBook Pro anyway) the [Option] key also has the characters alt on it, so I think of this as an alternate function, and it's not hard to remember.