Bailaman reports  that to counter the incumbent President Mahinda Rajapaksa's immense popularity after defeating the Tamil Tigers, “the main opposition party in Sri Lanka (UNP) has formed an alliance to contest at the presidential elections. Led by UNP the new alliance will contest as the United National Front.”
There is a twist in this candidature as per Bailaman:
Interestingly Ranil Wickremasinghe has pledged to abolish Executive Presidency in Sri Lanka  if his party wins. With his new set of conditions which requires Fonseka to agree to Ranil Wickremasinghe as PM, Fonseka's role in the governance of Sri Lanka will be minimal.
However Dzone reports  quoting the Sunday Leader that:
Reliable sources who spoke on condition of anonymity said Fonseka has categorically stated that he will not abolish the Executive Presidency. Abolishing Executive Presidency is a vital component of the campaign spearheaded by the United National Alliance.
Although the presidential election is not due within 2 years, rumors are that Mahinda Rajapaksa may hold them as early as possible, predictably in early 2010 to take advantage of his current public support. Speculations are high in the Sri Lankan blogosphere and Dheshapaluwa lists 7 reasons  why Gen. Sarath Fonseka will run for President of Sri Lanka. To contest at the election Fonseka had to resign from his post in the Sri Lankan military immediately as the government is likely to announce election dates on Sunday, November 15, 2009.
And the expected happened today. Suranimala reports :
It is reliably learnt that Chief of Defence Staff General Sarath Fonseka has handed over his letter of resignation from the post of CDS to the President Office today. It is also reported that Gen. Fonseka has mentioned 16 reasons on his letter of resignation as to why he decided to resign.
Another fact may have induced Fonseka to resign. Sudha Ramachandran at Asia Times tells  how Sri Lanka was split over war honors as Rajapaksa's allies downplayed Fonsekas contribution in defeating the LTTE and even had launched a campaign projecting Rajapaksa as “Sri Lanka's savior”:
Fonseka was a big part of the offensive, leading crucial military offensives. He was army commander from December 2005 to July this year, leading the planning of the final phases of the “Eelam War IV”, which culminated in the final defeat of the LTTE.
The Rajapaksa government has tried to downplay his contribution. “It was President Rajapaksa and his brother, Defense Secretary Gotabhaya Rajapaksa, who steered the forces to victory against the LTTE,” Labor Minister Mervyn Silva said at a public rally recently, calling on Fonseka and others not to take “personal credit” for the victory.
Fonseka, an US Green Card holder, also was subjected to controversy recently as the US Home Security Department attempted to “interview him over allegations on war crimes in the last phase of the war”, reports Lanka Polity . Sri Lankan government objected  mentioning that “he has no authority to share any information relating to national security with third parties.”
Bloggers are also weighing in on Foneska's probable candidacy. Lanka Libertarian thinks  that this is a game of the opposition leader Ranil Wickremasinghe (UNP):
Ranil does not expect Fonseka to win if he is the opposition presidential candidate.
He is just fighting for his own political survival as unp leader in case of elections. He wants to revive the unp grassroots for parliamentary elections to prevent another dismal showing, and then hope to remain in control of unp after fonseka loses (or if there is no presidential election).
Indrajit Samarajiva at Indi.ca shows the irony of the situation :
The UNP traditionally includes minorities (though the riots were under a UNP government). They would now give northern Tamils the choice between the guy who bombed them and they guy who ordered the bombing. But stranger things have happened.
Lionel Bopage at Groundviews is apprehensive about another military chief turned democratic leader and poses some questions :
Under the circumstances, what guarantee is there whatever bourgeois democratic characteristics that remain in Sri Lanka will not be brought to an end by an army general elected as President of the country? Would not one want to remain President for life once elected? What are the safeguards that could be applied in such circumstances?