Like Tim says, it will import longer alarms from most services. I know from experience that Google Calendars allows you to set longer alarms and sync them. Using MobileMe online won't, but I can use the Calendar app on my iMac to set almost anytime and it will sync with both MobileMe and the iPad.
I did run into problem when I tried to set up more than two alerts though. MobileMe choked on it badly.
It's an annoying oversight. There really should be an option to set your own time and date.
The workaround is to create a reminder event on the date you want to be reminded of the future event. That is what I eventually did when faced with the three alert limitation.