The visa letter was replaced by the CAS Number (Confirmation of Acceptance for Studies) this Monday 22nd February. Anyone making a visa application on or after this date must include a CAS Number with their application, otherwise their application will be refused.
This includes people who have recently had their applications sent back to them because the application was invalid (e.g. the photos were not acceptable or a necessary part of the form was not completed). Even if you originally sent your visa application last month you will need a CAS Number if your application has been returned to you and you did not send it to the UK Border Agency on or before 21st February.
To request a CAS Number you must go to your Undergraduate / Postgraduate Office with your passport. Staff will probably not be able to give you your CAS Number immediately because they need first to apply to the UK Border Agency for the CAS, and then it is up to the UK Border Agency to produce and send your CAS Number.