GHCi, version 6.12.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> :i True
data Bool = ... | True  -- Defined in GHC.Bool
Prelude> :def ifoo \_ -> putStrLn "ifoo" >> return ""
Prelude> :i True
ifoo

It seems that abbreviations are resolved to the command, that matches the prefix, which is defined last. I've found this behaviour very confusing in practice.

I'd expect rebinding :i to some command that's not :info to require an explicit declaration.