Still an issue, though small. This is a warning we print for your convenience, you can ignore it if you want to continue to make mono files. Warnings can be ignored, errors will prevent you from publishing.
Mono mp3s will not play in older first generation iPods and other budget/low end mp3 players. It will also not play on computers that have older versions of Flash installed.
If you are using a high end audio editing product, you can save your audio in "joint stereo" and it should give you the same file size as a mono file. Anything using the LAME encoder to create the mp3 will split the mono into 2 channels and double the file size as you noted.
The warning is there so you know that if you do get a complaint from a listener, you know why. Before we had the warning we had numbers of support tickets from podcasters that would receive complaints from their listeners that their audio would not play.