Gallup's tracking polling this week reversed President Obama's recent entry into net approval territory (49 approval - 45 disapproval, Feb 6-8), back down into negative territory (46 - 48, Feb. 14-16). Detailed data available on interactive chart here.

These are ratings are somewhat volatile, but the trends no doubt are more significant than the short term fluctuations.

Bryan Preston at the PJ Tatler explains most of the negatives that could be affecting the drop into negative net approval:

we've seen what looks like the start of an early gas price spike, at a time when the president is visibly militating against the XL Pipeline. A majority supports building the XL (snip)

the public is aware that Obama's spurning of the Canadian deal has driven our ally into the arms of the Chinese (snip)

The president has also released a budget, but everyone knows that it's already DOA in Congress. Most of the press that budget has gotten has been negative (snip)

the president has turned his administration into a regime with the abortifacient mandate. 

High gas prices are a huge vulnerability for Obama.  There is worse to come since crude oil prices have been spiking. Only part of this is due to Iran.  If anything, Iran has moderated its language the last two days and crude is still spiking. The Fed is juicing the economy with ultra-low interest rates.  They have to do that (and flood the system with more cash) because Obama's policies have otherwise retarded growth. But it creates conditions for spikes in oil.

Richard Baehr adds:

He has lost no ground in this period in head to head matchups with Romney or Santorum. He is vulnerable, but our guys have limited appeal