XF is a great platform and for what it is $140 is an amazing price. In all honesty, a summer sale is very, very unlikely.
I can only imagine a limited time discount being offered if XF 2.0 is to cost more. Much like what IPB is doing right now.
Actually, with IPB (which in my mind is XF's main competitor) increasing their pricing, maybe that does give XF a little room to boost up their prices whilst still maintaining a more competitively priced system? Once the promotion ends IPB will cost $200 for the base product and $50 to renew for 1 year compared to XF's current $140 / $40.
Time will tell.