Just wanted to add to the above that Sergei B. Ivanov, who is one of the 5 contenders mentioned in the article, is currently serving as Foreign Minister. Gaia Man foresees the strong possibility that Putin will pick him to be his front man while he rules from behind the curtain. Here's more speculation on Putin's successor from "The Independent".

".. <snip> ...Putin is by far and away the favourite contemporary figure for Russian citizens, scoring a 75 per cent rating as "good" or "excellent" in a opinion survey conducted in July..<snip>...Above all, it is his success in striding out on to the world's stage that appeals to his fellow Russians. <snip>...His countrymen love his touch of menace..."

"..<snip>..There is a limit to Putin's power: the Russian constitution forbids him from standing for a third presidential term, and he has ruled out calling for a repeal. If Putin was able to stand again for the presidency, he would definitely win. No Kremlin leader since the 1960s – Khrushchev, Brezhnev, Gorbachev and Yeltsin – was much missed when he left office. Putin is likely to be the exception. The frontrunners to succeed him as President in April 2008 are Dmitri Medvedev and Sergei Lavrov. Both are First Deputy Prime Ministers. Medvedev has only an 18-per-cent rating from the public. Lavrov manages only 14 per cent. No one doubts anyway that it is Putin who will choose his successor. Putin will surely find some way to go on influencing affairs from behind the scenes."

[link to news.independent.co.uk]