We too are going in 08 and not renting a car. I kinda figure it will work this way, from what I read here...
fly into airport (LAX or SNA) and get shuttle to
Disneyland walking distance hotel, spend days at Disneyland, nights at hotel, walking everywhere.
Take amtrak to Carlsbad
Legoland (Oceanside station), taxi or bus to hotel then same to legoland (not sure what to do with luggage so thinking would have to go to hotel first, maybe there is one walking or short cab distance or one that has a shuttle...
spend night in Carlsbad. Take morning amtrak to San Diego (SDS station). Again, taxi or bus to hotel then same to SeaWorld or zoo. Spend 2 nights in area so we can do both SeaWorld and Zoo, taking taxies/shuttles/busses to and from attractions.
Last day fly out of San Diego OR amtrak back up to Anaheim area and shuttle to airport. Prefer 1st option.
I think if we do this, it will work. The only issue is the luggage on the days we do the Amtrak, so not sure how that will work. Even with the shuttles, trains, etc. it should work out cheaper than renting a car.
The OTHER option is to not rent a car for the Disney part of the trip, but only for the Legoland/San Diego part. That would make the rental only for 3 or 4 days.
Just my vague plans from someone who has never done this before... LOL.
Nora