There really is a problem on (at least some) cable upstreams today, based on what I hear from people I respect who have the data. My hope - which won’t be tested until 2009 - is that the DOCSIS 3.0 upstream will resolve most or all of the problems for the next few years. Full DOCSIS 3.0 has a minimum of 120 megabits upstream (shared) among typically 300 homes, something like 400K per subscriber. Current cable modems typically have 8 to 30K per subscriber. This is a huge difference.
While those 'K' don't indicate whether those are kilobits or kilobytes, a bit of quick math tells us that those are kilobit counts. In other words, currently Comcast is allocating a minimum of 1 to 4 KBps for each subscriber. As well, IIRC, Comcast sells 384 to 768 Kbps upstream connections. That puts the overselling ratio between 13 and 100.
Another section is also interesting, for comparison with DSL and FIOS:
Verizon, AT&T, and Free.fr are strongly on the record they do not have significant congestion problems. They do not have a shared local loop, and have allocated much more bandwidth per customer. I’ve sat at the network console of a large ISP and seen they had essentially no congestion amongst the millions of subscribers they served. They allocate 250K per subscriber, much more than current cable.
It's not clear who these figures are for. I believe AT&T DSL doesn't offer more than like 768 Kbps upstream, in which case this would be an overselling ratio of 3. If this is Verizon FIOS (let's say at 5 Mbps, which is their faster speed), that's an overselling ratio of 20. Suddenly it seems very unsurprising that Comcast is having problems and AT&T/Verizon are not. It also shows you who's been investing in their network over the last decade and who hasn't.