First, download and listen to Episode 45 of the
Sex is Fun Radio Show; "Strap on Dildo Show". There's lots of good information in there, especially for how to broach the subject.
A couple quick points from there: do you really "know" she will like it? Have you done any anal play together? I think these things are better discussed outside of the bedroom, rather than throwing it at her unexpectedly and hoping she'll go along.