I wrote a few months back about web engineers worldwide fighting to stop the advance of the false notion that using getElementById() shortcuts is a good idea. Apparently some scripters have only just discovered the IE trick and are now encouraging everyone else to dump the proper DOM methods and write malformed code.
http://www.raizlabs.com/blog/2006/12/no-more-getelementbyid.html
Real browsers such as FF will accept it if you have JavaScript settings set a certain way, but traditionally this 'shortcut' has been the primary cause for scripts failing on other browsers when written for IE, because it was IE which propogated the idea that it's valid Javascript.
It's also a bad idea to rely on implicitly available DOM elements from this position in the variable space, for the same reason that you shouldn't use Magic Variables in PHP… albeit slightly less of a risk in Javascript.
Argh.
