I know that the purpose of King's Cross Thameslink after December 9 has
been discussed at length on Usenet before, but it looks like TfL have
finally published some information related to this issue on the KXSP
project pages.

Shortly after the last FCC service departs from KCM at 01:06 LUL will
claim ownership of the station, which thereafter will be open
07:00-20:00, Mon-Fri. They do not say whether they will man the ticket
offices and/or if they will install any self-service ticket machines
there. (http://tinyurl.com/39lu4f)

According to the local press in Hertfordshire, First Capital Connect
will name one of their trains 'King's Cross Thameslink' at Brighton on
December 5 and another 'St Pancras International' at Bedford on December
9; in addition, the Transport Secretary will 'officially' open the new
Thameslink facilities at London St Pancras in front of 100 guests on
December 10. (http://tinyurl.com/399aly)

Almost forgot - TfL have just published a Nov07-Apr08 newletter about
NTH works progress at KXSP on the project pages. (http://tinyurl.com/38ftxu)