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I quickly installed iMenu (3.1) in 2.5.1a and other than having to change 'TYPE=MYISAM' to 'ENGINE=MYISAM' in ./sql/mysql.sql it appears to work okay. There's some PHP NOTICEs about Constants being defined more than once in admin, but that won't actually impact operation...
I didn't really look at it any further for any other problems, security, etc. but it does install and 'appear' to work with the one change mentioned above.