Russian Olympic team
After the latest Fed Cup matches I think Sharapova has established herself as the leader of the Russian Olympic team for Beijing. This is quite a turn around, considering that she was on the fringe not long ago. Shamil Tarpischev is a genius in his persistent courting of Sharapova since last year. He knows Maria is a true contender who can bring home the medal, regardless of her motivation to play for Russia.
Anna Chakvetadze has also established herself as a strong member of the team who can step up to extraordinary circumstances when needed. As a plus, she gets along with Sharapova so team harmony is not a problem.
With the two singles spots filled by Maria and Anna, the situation becomes interesting. Kuznetsova definitely has to be included because of her high ranking and previous achievement. After that, there's not a clear 4th wheel.
Dementieva - Not in good condition and not sure she would want to be the 4th wheel with Sharapova as the leader. Has a cold relation with Maria in the past.
Safina - Has proven unreliable and mentally weak, and has not contributed much to Fed Cup.
Petrova - Not in good condition and known to be mentally weak at critical moments.
Kudryatseva, Vesnina, Dushevina, etc. - Not medal contender whatsoever.
I think this leaves Maria Kirilenko as the 4th wheel. This is a good choice because Maria K gets along with all other 3 members on the team. Moreover, Kirilenko has actually won a tournament in Beijing so she will feel home there. She can also pair up with Sharapova to play doubles, although I guess Tarpischev's plan is to have Chakvetadze+Vesnina.