As of Ionic2 & Angular2 Beta the right path is:
import * as Rx from 'rxjs/Rx';
Note that as of latest version of Angular2/Ionic2 uses RxJS v5.0.0-beta, so be sure to read the migration readme as some of methods names has been changed.
If anyone interested in working example of Ionic2 Beta RxJS implementation, I made a chat example app available here.