It is a good question what payment gateway to start with. PayPal is definitely #1 but I am not sure if they support this functionality.
We will start adding this new service for major gateways first where we support tokenization so subscriptions are possible.
This change is definitely not going into v2.2 we are going to release by the end of the month so my ETA is "this year" in 2.3 or 2.4.
Yes, I am going to IRCE this year, see
http://blog.x-cart.com/X-Cart-Team-at-IRCE-2015-Chicago-June-02-04.html
But I think you might be interested in joining us at the US road trip we are planning this year in September - see
http://forum.x-cart.com/showthread.php?t=71554
I see you are from CA, USA and we are planning to visit SFO or LA during the trip.