MSDN documentation indicates that cookies are case insensitive. After spending hours to fix an ugly ASP code without the original programmer being present, finally found out that in 2 places a cookie is set as "strXyzUser" and in one place it is read as "strxyzuser". Works most of the time, but in some IE 5.5 installations, it fails. Changed all code to use "strXyzUser" and it all started working fine.
Just experienced the same thing but with various versions of IE6, and it may depend on the upgrade path. Some users of our software were getting an error when certain cookies were being read/set. I was not getting it with 6.0.2800.1106.xpsp2.030422-1633 Cipher Strength 128-bit but others who claimed to have the exact same version did. Only difference I can figure is that my PC was a clean build of XP whereas others were likely upgrades from Win98 or W2K.
Bottomline: be safe and assume that cookie variables ARE case-sensitive!