it looks like we really need to use

<a href="foo.php?a=1&amp;b=2">click me</a>

that is, the "&amp;" instead of "&"

using "&" works in most cases except that some Facebook package will not take it as valid XHTML... so I am forced to use "&amp;"

however, I am worried that using "&amp;" may choke some browsers like IE 6 or even IE 5.5? Or does any one know that it is well supported by most browsers? Thanks.