It seems to not, er, be true:

During his presidential campaign and subsequent battle over a health care law, Mr. Obama quieted crowds with the story of his mother's fight with her insurer over whether her cancer was a pre-existing condition that disqualified her from coverage.

In offering the story as an argument for ending pre-existing condition exclusions by health insurers, the president left the clear impression that his mother's fight was over health benefits for medical expenses.

But in "A Singular Woman: The Untold Story of Barack Obama's Mother," author Janny Scott quotes from correspondence from the president's mother to assert that the 1995 dispute concerned a Cigna disability insurance policy and that her actual health insurer had apparently reimbursed most of her medical expenses without argument.

A lot of the right is no doubt going to jump on Obama as a liar, but I think it's entirely plausible that this is how he remembered it: it was a terrible time, and memory is unreliable.  I just called a source to verify a story about the passage of the steel tariffs under the Bush administration--only to find that despite my crystal clear memory of the story, it actually concerned the 2002 Farm Bill.

However.  That's why I called my source: because memory isn't that reliable.  Given how often Obama rolled out this claim on the campaign trail, I think he should have checked the details (or had one of his staff do so).  Repeating it over and over, without ever looking into it, leaves you vulnerable to the all-to-plausible charge that this story was, as journalists like to say, "Too good to check."