I've only seen this if the mail hasn't been sent yet and it's rightfully showing the file as in use. Either save to a different name, or if the mail is unsent remove it from the email and re-attach after saving.
Here's a weird one: Often when I save a jpeg out of PS CS4, I have to quit Mac Mail or I get a file busy or in use message. It only happens if I've attached the image previously into an email. After I quit Mac Mail, I can save the image as a jpeg. Anyone else have this problem?