O2 have said today that they will only sell the iPhone 4 in their retail shops to
existing O2 customers only (mobile, broadband or homephone) initially.
O2 also won't be selling the iPhone 4 online until at least the end of July, where it will then also be available to everybody.
Details
hereAlso, from Monday 21st, O2 retail shops will be able to give out iPhone 4 micro-SIM cards ready for the 24th. If you have purchased an iPhone 4 direct from Apple, they will need to see a copy of your Apple order confirmation.
Once you have a micro-SIM you can then start the number transfer process by filling in
this form which should take about 24 hours once you activate the iPhone 4/micro-SIM via iTunes.
I have no idea how other UK networks are planning to implement the SIM card charge over.
O2 have already said that iPad and iPhone 4 micro-SIM cards are not interchangeable.