In short: The press was relatively cautious in reporting on this story. Finally (by June, 2013), limited ("small scale") use of sarin by the Assad regime against the rebels was confirmed by the US intelligence community. A lot is riding on the answer to this question at the moment (Western support for Syrian rebels for example), and I suspect it will be difficult to extricate facts from politics going forward.
Dan Roberts from the Guradian filed the following report on May 2, 2013:
Western intelligence agencies fear they can no longer prove for certain whether the Syrian government was responsible for alleged chemical weapon attacks, because initial samples and evidence trails have degraded over time.
Instead, Britain and the US are likely to have to wait for fresh evidence from further attacks before deciding whether to take a military response against the Assad government.
On May 6th, 2013 BBC reported:
UN's Del Ponte says evidence Syria rebels 'used sarin.' Testimony from victims of the conflict in Syria suggests rebels have used the nerve agent, sarin, a leading member of a UN commission of inquiry has said. Carla Del Ponte told Swiss TV that there were "strong, concrete suspicions but not yet incontrovertible proof".
The BBC reported (April 23, 2013):
A senior Israeli military official has said Syrian forces have used chemical weapons against rebels several times. [...] He said photographs of victims foaming at the mouth and with constricted pupils and other unspecified symptoms "provide evidence that deadly chemical weapons have been used".
There has so far not been any confirmation that chemical weapons have been used during Syria's two-year-old conflict although there have been numerous accusations. [...] A UN team is waiting for permission from Syria to enter the country to investigate the claims.
On June 4th, 2013 Reuters reported:
The U.N. commission said it examined four reported toxic attacks in Syria in March and April but could not determine which side was behind them.
"There are reasonable grounds to believe that limited quantities of toxic chemicals were used. It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator," Paulo Pinheiro, who chairs the U.N. commission of inquiry, told a news conference in Geneva.
On June 13th, NPR reported:
Deputy National Security Adviser Ben Rhodes said that an estimated 100 to 150 Syrians have been killed in attacks using sarin gas, although the figure "is likely incomplete." "Following a deliberative review, our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year," Rhodes said in a statement. "Our intelligence community has high confidence in that assessment given multiple, independent streams of information."
Barring further major developments contradicting the above, I consider this question resolved in the affirmative: according to reputable (American) sources Assad used chemical weapons against his opposition in 2013.