While I was living in Japan, I did a lot of travel around Asia.
In general the customs and immigration folks in Taiwan, Hong Kong, Japan, Thailand, Australia, and Korea are very polite and I've never been hassled. Yes Australia is not Asia exactly, but 10 hours to Australia from Japan is a heck of a lot better than 17 hours from LAX.
However, 2 cautionary notes:
1) Due to SARS, I would not be getting on an airplane if I had any type of cough, sniffle, or even allergy. Going to any of those countries above plus mainland China means you risk a 10 day enforced airports 'SARS-batical'.
This is of course ignoring the SARS chance itself.
2) Don't ever carry Tic Tac type mints in any luggage - especially going to/from Thailand. I once had a pack of white Tic Tacs in a checked luggage coming back from Thailand. The only time Japanese customs ever checks anyone is for Thailand returnees - and the **** Tic Tac box had broken open and spread everywhere in the suitcase. Needless to say a bunch of small white pill-like objects in my luggage then engendered a 4 hour wait in the airport while one (or more) of said objects were tested for drugs.
Everyone was very polite, but I personally will only carry the dissolving film breath fresheners from now on. At least until I get hassled by someone thinking they are speed tabs...