You need to specify what mail client programs you will be using or will be targeting. My client, Eudora, doesn't seem to understand any of the subject lines I get in Chinese. (I made the mistake of using my real email address in Usenet chinese groups - bu xing, hen duo SPAM) However, I do see a _lot_ of things looking like:

Subject: =?Big5?B?pXjGV7DP?= email =?Big5?B?pua+UKRqpf4=?= &
 
Subject: =?GB2312?B?vKu+39T2s6THscGmtcSzr9H0uN+/xry8OqGw0tTIy8jPyMuhsQ==?=

Subject: =?GB2312?B?xOO6w6Oh?=

Subject: =?Big5?B?p1muybNxtfi2WrHQvsc=?=

Beg pardon at what these might actually say. ;-)

Google-wise, I see mention of similar constructs in http://www.ietf.org/rfc/rfc2231.txt
Yumm, check out RFC 2047 at http://www.ietf.org/rfc/rfc2047.txt
This second one is apparently the controlling RFC for what you are looking for.

I'm a pessimist about probabilities; I'm an optimist about possibilities.   --   Lewis Mumford