What is the mechanism for thermal decarboxylation of a generic carboxylic acid (not special cases like beta-ketocarboxylic acid or malonic acids)?
Under sufficiently alkaline conditions, i.e. when the acid is deprotonated, the carboxylate can undergo an electron transfer reaction with a suitable partner (oxidant). In the course of this process, the carboxylate is oxidized to an acyloxy radical, which subsequently fragments to yield an alkyl (or alkylaryl) radical and carbon dioxide.

$$\ce{R-COO- -&gt;[-e^-] R-COO\cdot -&gt; R\cdot + CO2}$$

A photochemical variant of this reaction was intensively examined in the group of Axel Griesbeck at the University of Cologne, Germany. For further reading, have a look at 


Acc. Chem. Res., 2007, 40, 128-140 (DOI)  
J. Phys. Chem. A, 2006, 110, 3356-3363 (DOI)
Synlett, 2004, 2347-2350  (DOI)