Just had a look at the iphone terms on the O2 web site and found this, it appears to indicate that customers can break the existing agreement without penalty and to move to a new iphone contract for min time (ie 18mths or whatever it is at the time). An unusual t&c as all other networks will enforce the min terms! Looks interesting and might give some confidence that you are not locked in a 18mth contract when apple launch (or might not) the iphone2...going to check with O2 customers services and get this in writing before it disappears..
http://www.o2.co.uk/termsconditions/iphone
8.3 If this Agreement is ended during the Minimum Period, you may be required to pay us the monthly subscription charges up to the end of that Minimum Period. This does not apply if you end the Agreement for the reasons in paragraph 8.4 or if you purchase a new iPhone from us, but in this case you agree that a new Minimum Period will apply.
8.4 You may end this Agreement by giving us written notice if:
(a) we break this Agreement in any material way and we do not correct the situation within 7 days of receipt of your written request;
(b) we go into liquidation or a Receiver is appointed over our assets; or
(c) we increase charges for calls, messages or data that form part of your inclusive allowance or your Line Rental Charges, or change this Agreement to your significant disadvantage, in accordance with paragraph 9.2 of the General Terms, provided you give us a minimum of 30 days’ written notice (and provided you notify us within one month of our telling you about the changes). This does not apply where the increase or change relates solely to Additional Services in which case you may cancel, or stop using, that Additional Service.