HTML has always allowed authors to use upper or lower case letters for element and attribute names. The next version of HTML, code-named Voyager (and sometimes referred to as 5.0), will be an application of XML, and as such will be case sensitive. Because of this new constraint, the W3C must specify either all uppercase or all lowercase letters (else the DTD would have to double in size to support both <html> and <HTML>, and it seems they don't want to do that).
Although the W3C has made an early decision to only support lowercase tags, many people never got the chance to indicate their preference, so I have prepared a poll to gather additional input. I will publish the results in the hope that the W3C will reconsider their decision.