In general this completely baffles me, particularly at non-hub airports where the information is known well in advance.

One particularly egregious example is CDG, where the roads leading to the airport do not even list airlines and most taxi drivers do not seem to know which terminal is required for United. (For the record, United uses terminal 1 at CDG, which seems to be where most star alliance carriers are.)

What's amazing is that even if you think ahead (since it won't be written on your boarding pass or flight itinerary) and try to look up the information on-line the day of departure, you still can't find this information anywhere on United's web site. As an example, checking the flight status of UA991 (which departs from CDG shortly) returns "Check-in terminal: [blank]".

This lack of information has to cause major headaches for a non-negligible number of travelers, and yet it would be so easy for United to fix. For $45/year, they should offer a service that always prints the terminal on your boarding pass. Make it $30/year for gold members, and free/automatic for credit card holders, platinum+, and GlobalFirst pax. Such a program would easily pay for itself. Think of the add campaign: "For only $45/year, get the security of always knowing where to check in for your flights. This offer will pay for itself the first time you don't miss a flight or avoid having to take a taxi between terminals. No other airline sells that kind of security!" Who would refuse such an offer? To sweeten the deal, make it apply to all boarding passes on the same PNR.