I just noticed that in the new UI of Google Spreadsheets, you can not only copy a page (or sheet) to a copy of itself within the same workbook (collection of ‘pages’ in one ‘file’) but you can now copy a sheet all the way into an entirely different notebook.
Click the menu arrow in the sheet you want to copy and click "Copy to…"
Then a pop-up appears allowing you to browse where you want to drop the copy:
Really. This is pretty killer.