My tuppence worth:

Sky claim that "a selection of channels" will be available from mid 2008:
http://skytv.custhelp.com/cgi-bin/skytv.cfg/php/enduser/std_adp.php?p_faqid=532&p_created=1178752328&p_sid=adTbIEZi&p_accessibility=0&p_lva=216&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9MiZwX3Byb2RzPSZwX2NhdHM9MCZwX3B2PSZwX2N2PSZwX3BhZ2U9MSZwX3NlYXJjaF90ZXh0PWhkdHY*&p_li=&p_topview=1

Looking at the link you will note that they completely fail to anaswer the question they have set themselves - ie "Will there be a charge". I think we can assume from this that the answer is 'YES'. The obvious question is then "How much?".

I love sport and would dearly like to see it in HD, but Sky will be causing themselves all sorts of problems if they try to charge too much extra for HD and/or do not provide the FTA channels in HD. I would expect a lot of people to share rugrat's view and vote with their feet.