For completeness sake, here is a report of it working beautifully:
# cat .dovecot.sieve require "fileinto"; if header :contains "X-Spam-Flag" "YES" { fileinto "spam"; }
if header :contains "Subject" "テスト" { fileinto "nihongo"; }
Subject: =?ISO-2022-JP?B?GyRCIVolSyVlITwlOSVqJWobKEI=?= =?ISO-2022-JP?B?GyRCITwlOSFbGyhCcGFwZXJib3kmY28uIBskQiQsJS8laiUoJSQbKEI=?= =?ISO-2022-JP?B?GyRCJT8hPCRyQmdKZz04ISohViVaJVElXCUvJWolKCUkJT8hPCU6GyhC?= =?ISO-2022-JP?B?GyRCJTMlcyVGJTklSCFdMkYkQCEqRy4kJCQ8ISolLyVqJSglJCU/GyhC?= =?ISO-2022-JP?B?GyRCITxBNDB3PTg5ZyEqIV0hVzMrOkUbKEI=?=
(That's the string:【ニュースリリース】paperboy&co. がクリエイターを大募集!「ペ パボクリエイターズコンテスト−夏だ!熱いぜ!クリエイター全員集合!−」開催)
Sep 11 11:46:22 test-vmx01.unix dovecot: [ID 583609 local0.info] deliver(user1@domain): sieve: msgid=20090911024611.C7348232223@domain: stored mail into mailbox 'nihongo'
Perfect!
Lund
-- Jorgen Lundman | lundman@lundman.net Unix Administrator | +81 (0)3 -5456-2687 ext 1017 (work) Shibuya-ku, Tokyo | +81 (0)90-5578-8500 (cell) Japan | +81 (0)3 -3375-1767 (home)