Aaron, a 'quick skim' is not at all enough to get a thorough understanding about the ports! Why not reading the following hardware description (from page 3) 'slowly' and 'carefully'?

http://www.semiconductors.philips.com/acrobat_d...WARE_1.pdf

By the way, it's not so difficult to understand this port stuff if you try to think about how you would manage it to combine an output and an input by using only one port pin. Think about that such micros would always try to implement TTL-compatibility. This means a simple NMOS FET (CMOS technology assumed) at output to fabricate logical low state. What's then the only option to make a port to work like an output or input? And how would you initialize the port to make it an input?
What I want to tell is: This active pull-up stuff, with a strong one being only activated for two oscillator periods and the two weaker ones being activated in the manner perfectly described in the above link is a logical consequence of the try to combine input and output facility by using only one port pin. There's nothing magic...

Kai