I found the SalesForce Component on the Xamarin components store here:
https://components.xamarin.com/view/SalesforceSDK
However there is no mention of MobilePush support. I'm actually only interested in how I might register for notifications with SalesForce after I have the token from Apple APNS and Google GCM. i.e. use a Rest or SOAP API to perform the registration rather than use an interactive SDK.