Everyone in tutorialworld seems to be obsessed with CREATING multi-resolution .ico's to use as favicons. My question, however, is what browser support for these multi-resolution .ico's is like. Will the common browsers (Chrome, FF, Safari, and IE7+) treat an .ico containing two resolutions (16x16 and 32x32) in some sane way? The consensus seems to be that going larger than 32x32 starts to bloat the file size without providing much benefit (except to IE9/Windows 7 and whatever they do with 64x64 images), at least.
There's a lot of fairly good information on wikipedia, but not about size support specifically:
http://en.wikipedia.org/wiki/Favicon
However, I don't think most web browsers have a use case for an icon over 16x16. I certainly haven't seen a large icon, but I'm not surprised since larger icons would tend to expand url-bars, bookmarks and tabs to the point that they take up prohibitive amounts of vertical screen real-estate. I wouldn't bother until you knew somewhere it would be used and decide that the bandwith is worth it.
One thing to look into is whether or not they are utilized by any accessibility features for low vision users (this would make sense), but the screen shots here (for example):
http://www.accessfirefox.org/Firefox_Accessibility_Themes.php
seem to show the same old 16x16 icon despite the increased font size. I suspect the fact that one cannot rely on the existence of a 32x32 resolution means that nobody spends the time to program in code that looks for it.
As I understand it this multi-resolution feature for icons is intended to target reuse in "other contexts" such as Prism (defunct) or Air (proprietary)