The Tivo uses the Original Airdate field in the guide data to know what to record. I believe the date has to be within 10 to 14 days of the Original Airdate in-order for it to be marked as First Run Episode. This is why Stargate SG-1 and a FRO SP missed the second half of Season 5 StarGate SG-1. (The last season on Showtime) since they aired the episodes in the UK about 3 months earlier.

The 28 day rule, if TiVo has already recorded an episode of a show within the last 28 days then it will not record it again unless you set the SP/ARWL to record all episodes or if the show only has generic guide data like South Park the past few weeks.

You should also know this: If you setup an ARWL for "The Wire" set to record FRO you have 14 days from the Original airdate to get the episode. This is nice because if you know of shows that are Shared-Window/Repurposed Network Television it can help you with conflicts.

SP = Season Pass
ARWL = Auto Recording WishList
FRO = First Run Only