Safe Harbor & The Data Protection Act
If Facebook have opted to abide by the Safe Harbor arrangement then they have agreed to comply with an EU Directive on personal data and not the Data Protection Act.
It is against the Data Protection Act to retain personal data for longer than is necessary. If Facebook was registered as a Data Controller in the UK then I guess that being unable to delete your account would result in a breach, as it could be argued that Facebook were keeping data longer than is necessary.
I've checked on the ICO's site though and Facebook are not on the register of data controllers, so I am unsure as to the grounds for the ICO to investigate.