I don't know who said it but it seems to be true.
ubuntu is African for "I'm to stupid to install Slackware"
IMO start with FreeBSD. HTML 5 is out or about to be released.

WEB Programing is always fun.

PHP, MySQL, PERL, MSSQL 2008 Studio if you're in a windows shop environment.

Always good to know a bit about dotnet framework

Everything you knew still applies. The new stuff is based on the old.

64bit hardware now means you can't write assembly anymore by doing the math in your head. At least I can't.

MetaSploit and Backtrack have some new exploits and tools.

Wireless has changed in the last four years.

Pretty much all I can think of at the moment.