How to mass convert RAW images into JPEG in Lightroom 3?

In Lightroom, open the catalog with the photos, select the collection or filter so that only the files you are interested in are shown in the library grid (or undo a filter/collection if you want ALL files).

Select all the files by choosing Edit | Select All (CTRL+A on Windows).

Export as JPG by choosing File | Export (CTRL+SHIFT+E on Windows).

In the Export dialog, choose your output folder, whether you want the JPG files added back to your catalog, and the output settings (resolution, sharpening, etc...)

And you're done!

(Note, I have Lightroom V2.7, but I suspect this process is the same in Lightroom V3.x).

I think I have a good workaround. Of course a plugin for this task would still be the best.
I did this with Lightroom 5.

1. Add the photos you want to convert to the (empty!) Quick Collection and select them all.
   You could also just select the photos without adding them to any collection. But then it's VERY important that you don't (accidently) unselect any of them. Of course this only works in a single folder. If you use a collection it works with multible folders at once.

2. Click "Export" an enter the following settings:

   Export Location
    Export To: Same folder as original
    Put in Subfolder: UNcheck
    Add to this catalog: check
   

   File Renaming
    Rename To: UNcheck
   

   File Settings
    Your decision
   

   Image Sizing
    Your decision
   

   Meta Data
    Include: All Metadata
    Remove Location Info: UNceck
    Write Keywords as Lightroom Hierarchy: ceck
   

3. Click "Add" on the bottom left to save the settings as a preset, so you don't have to enter them again the next time.

4. Click "Export" and wait.

5. Wait a bit more. Drink some coffe, read a book, take a walk and shoot more RAW files.

6. Go to the Quick Collection and delete every file in it or delete every selected file if you didn't use the Quick Collection. The JPEGs won't be added to the Quick Collection (or any other collection) automaticly, so don't worry to delete one of the converted JPEGs.