.Rtf Documents Being Used in Spam Campaign

Not as interesting as it is unusual, the .rtf file (rich text format) is being used as a spam vehicle in a rather large blast currently. The emails themselves contain a news headline style subject line and are blank except for an attachment of the .rtf variety. The name of the files vary, and they are all around 363 bytes in size. The documents themselves don't contain any exploits, just a single sentence advertisment once they are opened up. The ad reads "http://XXXX.org - Order the cheapest medications now!"Where the "x's" are numbers that seem to be different in almost all of the samples I've peered into.
This is a campaign I'd usually not think twice about, but it is rather large. Since blocking them about 10 minutes ago, we have already caught over 400,000 pieces, and the numbers are still climbing.