Part of the problem is that for years the networks have been heavily promoting their networks and the mobile internet - paired up with the infamous "unlimited" data plans that have caps and fair use clauses. Now that people are actually using the internet from their smartphones, and in significant volumes, the network infrastructure is starting to struggle.
While the O2 problems were (mostly) limited to London, they are addressing the problem at a national level with more efficient infrastructure, an increase in the number of base stations and "learning about applications that could place heavy demands on the network."
And there is human nature. We don't want to pay in our mobile phone bills, but we also want to be doing more with our phone every day. It's unlikely that people who have been heavy data users will start to use less data.
-- Ewan Spence, Dec 2009.