UPDATE (3/31/2011): Asad below in the comments offered this document as a trick to trick the toner and actually "reset" it. I downloaded it and am putting the direct link here because I noticed the site was kind of laggy. http://www.jeffersonkim.com/other-files/BrotherTonerRefill.pdf?attredirects=0&d=1
UPDATE (3/28/2011): I just buy them from here now and don't seem to have any problems: http://www.meritline.com/newsearch.aspx?SearchTerm=brother+tn360
If you get this message, try putting a piece of masking tape on the toner cartridge hole on the right side. It's a translucent, white, circle that the printer "looks" through to detect how much toner is in the cartridge. You usually can extend the life quite a bit until the toner actually runs out.
If the error message remains, even when you replace it with new, re manufactured toner cartridges, then you'll need to bite the bullet, and buy an "authorized" toner cartridge.
At least, that's what I did to make the error message disappear. Then, I just switched out my old toner cartridge to make it work again.