Read this interesting article to see how the guys at Mainsoft optimized the conversion of numbers to strings and vise-versa by more than 2.6x. Their goal was to leverage the performance of text-based internet protocol implementations such as XML and HTML.
The article also introduces a valuable comparison to conversion algorithms in .Net. Java. It's a good reminder for all of us to keep looking for improvements and stop taking things for granted. The source code is available too.