Up until yesterday, there had for all intents and purposes only been two Linux distributions with real traction in the enterprise space; Red Hat and SuSE. Some of that is sales hype and public perception of it, but a good part of it was real. RH and SuSE were the only distros with real, professional support capabilities. Organizations running other distros (and there are many) either bought support from third parties or did it all themselves.
Yesterday, Sun announced they would partner with Canonical, the parent company of Ubuntu, in porting their Linux to UltraSparc T1 "Niagra" multi-core cpu. This is a HUGE win for Canonical and probably adds Ubuntu to the short list of "top tier" distributions with RH and SuSE.