It's been discussed here a bit, and LATS isn't stepping in, so I'll repeat what he and others have said so far.

Basically, the only problem is whether the speaker can handle the low frequencies. The actual amp will have no trouble, and that's just plain logic; you're not driving the amp any harder than with a guitar. However, the speaker might not like the extra lows.

However, a guitar into a bass amp is no problemo and was done with great success in the olden times.