According to an internal source, the online payment company Paypal today initiated a mass chargeback on "thousands" of Sony Entertainment Network transactions, for reasons yet-unknown. These chargebacks have ended up putting the PSN accounts affected into debt, which triggers the system to auto-ban that account until it's back in the black. At the moment this seems to only be an issue for UK-based accounts.
There's an easy way to tell if you're affected: check your email. Those who are should find an email from Paypal about a refund from transactions relating to Playstation.
Understandably, the relevant call centres are rammed with people trying to figure out what's happened. So: if your Playstation account has been suspended, this is probably why.