[desadevil]

I was wondering if anyone could tell me how cis-trans isomerism affects acidity?
For example, how does the configuration of maleic (cis-butenedioic) acid render it higher in acidity than its trans counterpart (fumaric acid)? Since the inductive effect explanation doesn't seem to apply here...
Thanks in advance! 

[Jorriss]

My guess is in the cis configuration you can get some intramolecular forces going on.