Gamasutra has compiled a fascinating and pleasingly colorful set of graphs highlighting the latest known tie ratios, both for first- and third-party titles, of the PlayStation 3, Xbox 360 and Wii. As the article notes, tie ratio can used to gauge the "health" of a system, with a higher number indicating more games being sold for every system escaping its retail self.
Comparing each system within their first 23 months on sale, the Xbox 360 had the highest third-party software tie ratio at 5.2, followed by the PS3 with 4.4 and the Wii with 3.1. The Xbox 360 has since gone on to establish a combined first- and third-party ratio of 8.1 after 35 months on the market. Interestingly, the Wii's ratio has remained quite consistent throughout 2008 -- certainly a positive result in the face of continued success in the hardware arena. Remember, a high attach ratio may sometimes indicate diminishing hardware sales, with consumers spending more on software as a system matures.