Given that this can affect anyone from a web page, any coffee shop which uses "n" is vulnerable. And I know of a few I frequent which use 'n'.
If this is 'n'-only, would going down to 'a' or 'g' mitigate it completely? Especially in a coffee shop where the outbound connection is unlikely to be very fast anyway, the bandwidth difference probably won't be noticeable.
Since 802.11n was the first standard to support packet aggregation, then dropping back to a/g will probably work, unless clients cannot be told by the AP to not treat the network as a mixed a/g/n network.