As some news sites are covering this as "news": This vulnerability has been long known in the field and was - to the best of my knowledge - first demonstrated to a public audience by Dieter Spaar at the Deepsec 2009 conference in November 2009. You can get his slides.
The difficult part for many years has not been to know about the possibility of this weakness. Anyone who has read the GSM air interface specification will inevitably see that there is a limited number of RACH slots and a limited number of dedicated channels. Once you fill more RACH slots than the cell has dedicated channels, and you keep re-filling them at a higher rate than the cell can expire those dedicated channels, you have a DoS.
So rather, the difficult part was to implement it in practise, as traditionally all GSM baseband chipsets have been extremely closed, just like the very software (firmware) running on them. Today, starting from Q2/2010, it is very easy to do a proof-of-concept implementation, as we have created OsmocomBB: An Open Source baseband firmware.
Dieter Spaar's implementation predates OsmocomBB development by the better part of a year. At that time, he had to resort to binary-patching existing proprietary (binary-only) baseband firmware. So I think people should recognize his effort in doing the first practical implementation of that attack.