Not sure about the Mac version but I have The Gimp2 installed on a WinXP laptop and have known it to baulk at saving a file with "Save as...". I get a gimp message box saying the save failed because of "Unknown File Type".
If this is the same problem you're having, it needs you to tell it what file type you wish to save it as. You can do this by typing the apropriate extension (.jpg, .tif etc.) after the file name in the name field or select a file type from the drop down list by clicking "Select File Type (By Extension)" then it should save the file.
Incidently, when you get past this part of the operation, another "Save as JPEG" window opens (assuming you've chosen Jpeg as the file type). Click the 'Show Preview in image window' box and the 'File Size:' dialog is activated, telling you the size of your file in kb. You can then adjust the 'Quality' to get the desired file size - useful for images to be posted on the forum. Clicking 'Advanced Options' acceses a load more adjustable settings most of which I don't understand but I usually un-check 'Save EXIF Data' and 'Save Thumbnail'.
Hope this helps
-------------------- Chris
Better a bad day on the water than a good day at the office