Researchers at the National Center for Scientific Research in France created a stir recently when they found an anomaly within the 802.11b standard capable of dragging down wireless LAN performance to the level of the slowest user.
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.