I have been messing around with the new icon format for Mac OS X 10.5 Leopard, enormous (256×256) and are-you-blind (512×512) just kiddin’. The high-DPI of modern monitors have exposed problems in the current interface. Most interface elements are pixel based. These elements are scalable in a minimal degree. But now with the modern screens with high-resolution, the interface gets smaller and smaller, including the icons. Computer companies resolved this by enlarging their icons. Microsoft chooses the vector-way (Windows Presentation Foundation), Apple the pixel-pdf-way (Aqua). Both have their pros and cons. Rich pixel based icons tend to get rather large in file size, so some form of compression has to be applied in TIFF files or ICNS files. Lucky enough I’ve always designed my icons media-independent-minded (OS, web, print and packaging). A lot of program icons have to be redesigned/upgraded, so good times for icon designers on both platforms.
Some interesting reading material on this subject: