Task/ To Do List App

[CKisses]

Can anyone suggest a task/to do list app that integrates with the native calendar? I'm very big on to do lists & I have a great app (SeizeTheDay) but the only draw back is that I have to basically look at 2 different calendars: the native iPhone calendar w/ my appointments/meetings & SeizeTheDay for my tasks. (On the blackberry tasks showed on the calendar together with appointments/meetings).

It's not a biggie to deal with 2 separate apps but if I can avoid it, I'd like to...

Any help is appreciated....