Here's another wordplay-laden Manyōshū entry:

淑き人のよしとよく見てよしと言ひし吉野よく見よ良き人よく見
Yoki hito no/ yoshi to yoku mite/ yoshi to ihishi/ yoshino yoku miyo/ yoki hito yoku mi
Good people heard that it was good, looked well upon it, and called it good: Yoshino. Look well on it, good people, look well.

The story is that Emperor Tenmu composed this poem while at Yoshino with his six sons, after having them all vow not to wage war on each other.

To commemorate such a solemn and important occasion, I personally would not have composed a tongue twister. But that's why Emperor Tenmu was Emperor.

Obligatory linguistic factoid: while all of the "good"s and "well"s are variations or conjugations of the same OJ adjective (yoshi, ancestor to modern ii/yoi; yes, the pun on "Yoshino" is intentional, although that yoshi means "reed"), they're written many different ways.

The first "good people" uses the kanji 淑 ("noble", etc.), but the second uses plain old 良 ("good", "superior"). Most commentaries explain that the 淑 refers to the emperors and extremely high-ranked folks of old, leading up to Tenmu himself, while the second refers to less exalted gentlefolk -- like, say, the Emperor's six sons.

Two of the "wells" are written with 吉 ("favorable", "splendid"); I have no idea why this is. (Anybody?) The last "look well" is written using kanji for their phonetic value (when used to represent Japanese words): 四来三 (yo.ku.mi).

Oh, and when 淑 people said that Yoshino was "good", they used the kanji 好 ("pleasing", "likable", "beautiful").