The question is fundamentally one of physics. As asked -- and not particularly well asked -- the answer is "any non-zero speed", because if you're moving and the AP isn't, you will, eventually, move out of range. You answered the physics question -- you'd have to be moving pretty fast to be out of tuning tolerance. (and you aren't going to accelerate to that speed while associated.) The practical NE answer "depends"... what's the scanning rate of the client? Is it a static address, or do we have variability from DHCP? What's the sensitivity (i.e. range) of the AP and client? What's the terrain? How much traffic do you need to send/recv? It's not a simple "yes/no" question.
(This gets debated in racing circles every few months/years. Taking VIR as a case study (because I have), if you place your AP on the pit wall at the center of the front straight, your android/iDevice will be in range to associate and pass traffic for a few seconds before moving back out of range. If you place an antenna on the roof of the patriot building, you'll be in range for minutes - possibly continuous. 900MHz non-wifi equipment does away with the range and terrain problems.)