In less than an hour I was able to add Rx support to MassTransit, an open source ESB:
https://github.com/MassTransit/MassTransit/tree/master/src/MassTransit.Reactive
Update: As for why it's a good fit, they already had a Subscribe/Unsubscribe mechanism in place. Adding Rx support means that those subscriptions can now be composed together easily. For example, you might have two kinds of messages that share some CorrelationId. With Rx you can trivially Join() the published messages by that identifier:
var someMessages = bus.AsObservable<SomeMessage>();
var otherMessages = bus.AsObservable<AnotherMessage>();
var joined = from s in someMessages
join o in otherMessages
on s.CorrelationId equals o.CorrelationId
select new { s.Something, o.OtherThing };
joined.Subscribe(x => Console.WriteLine(x));
Also: Check out https://github.com/reactiveui/ReactiveUI for an Rx-powered MVVM framework targeting XAML (WPF, Silverlight, WP), iOS and Android. Very, very cool stuff.