In GIMP 2.8, the developers changed the way you save files. "Save" is now used only for GIMP's native format, XCF (and compressed variants like .xcf.gz and .xcf.bz2). Other formats that may lose information on layers, fonts and other aspects of the edited image must be "Exported" rather than saved.
This has caused much consternation and flameage on the gimp-user mailing list. I can't link to all the threads, but you can find them in the gimp-user archives starting in mid-July and continuing through at least August.
Hoping for a productive solution rather than more flame wars, I've written a quick save plug-in which will save to any format, including XCF. It marks the image as clean, so you won't get an "unsaved images" warning when you quit just because you've saved to JPG and not XCF.
I announced Save/Export clean on the gimp-user list on August 11, and wrote a longer discussion of my own workflow on my blog a few weeks later, GIMP: Re-uniting Save and Export.
The current version of the plug-in lives in
Save/Export clean. Click on "Raw", then save the resulting file
to your GIMP plug-ins folder. On Linux and Mac you'll have to make it
chmod 755 save-export-clean.py or the equivalent
from your file manager).
You must have GIMP Python working -- the plug-in needs to be able to display a file selection dialog so you can choose a filename if your image doesn't already have one, and script-fu can't do that.
It will show up in your File menu as Save-export clean, probably right under Save a copy.
You can even bind it to Ctrl-S, if you want. But that's somewhat dangerous since it makes it easy to overwrite an original JPEG. Here's how to rebind keys in GIMP: Creating Shortcuts to Menu Functions in the official GIMP manual. Personally I use the Dynamic method -- I find it much easier than the shortcut editor.
Warning: this plug-in does not have a lot of warnings or checks! If you edit a JPG and run Save-export clean, it will overwrite the original JPG file. It will do exactly what you tell it to do, without pausing to ask for confirmation.
With that caveat, I hope you enjoy Save/export clean, and I hope it helps your GIMP workflow.