From the HTML 4.01 Specification, published 1999, section B.2.2 Ampersands in URI attribute values:

We recommend that HTTP server implementors, and in particular, CGI implementors support the use of ";" in place of "&" to save authors the trouble of escaping "&" characters in this manner.

Six years later, and a quick test with Apache/1.3.33 (Debian GNU/Linux) and PHP/4.3.10-16 shows that the semi-colon is not supported as a delimiter yet, for that combination :(