This notion is actually old news and is known as the near/far problem, where clients closer to a wireless access point can block or diminish connectivity for clients located farther away.
Most well-engineered WLANs are not designed to provide 1 Mbps over their range of coverage. However, even where the near/far problem exists, degraded performance isn't a critical issue. The reality is that all users are sharing a 6-Mbps channel.
This is why the somewhat limited range of 5-GHz 802.11a, combined with many nonoverlapping channels, makes that technology so much more scalable than the 2.4-GHz-based 802.11b or 802.11g standards. You want small cells so you can reduce congestion and optimize performance. And you don't want to worry about co-channel interference.
It's one reason why the 802.11g hype is such a crock. Imagine running a new 802.11g network and being throttled back to 1 Mbps because of the near/far problem. The chip vendors are working on a packet-bursting solution that may overcome this, but it's all still proprietary.