Please take note, that they'll be a candelight vigil:
Venue : Dataran Merdeka , Kuala Lumpur
Date : Saturday, 27th September 2008 (RPK's Birthday)
(More info here)
You could send a card to him:
Y.M. Raja Petra Kamarudin
Tempat Tahanan Perlindungan,
34009 Kamunting, Taiping.
And if you are a Malaysian citizen and feel strongly against the ISA please sign the petition here
Edited to add:
There are rumblings in our nation of Hartal ISA. Please read more about it here.