(I posted this on Ovid's use.perl journal as well...)
I haven't taken a detailed look at Maypole yet (no tuits), but just off the top of my head there are a number of similarities between Maypole and OpenInteract (I'll focus on the still-in-beta OpenInteract2 because it's got a lot more going for it). Generally OI2 is much more heavyweight and designed to adapt to more environments than Maypole. But that doesn't mean it's better -- as usual flexibility comes at a price, in this case complexity. And as usual whether this complexity is worth it really depends on what you're doing.
There are almost certainly more differences, but I have a non-Perl day job to get back to :-) That said, I think the application server problemspace is sufficently complex that having multiple solutions are extremely useful.