My vote would be to add sms or email to sms and plain email reminders like that done on
www.airset.com (a free AJAX calendar site) as well as pop-up reminders. I use airset currently and would have to stay with it until something like this gets implemented because I love to have those reminders sent to my cell phone.
They are REALLY easy to implement too as most cell providers let you send an email to a certain address (like
number@message.alltel.com for alltel), then they convert it to an SMS message and send it to the phone. They are almost always instantaneous and great for reminders.
I would be happy even to take the first stab at implementing them if someone would point me in the right direction.