I didn’t see in that article where it confirm Stupak was going to attend. His only quote is:
“I had no prior knowledge of either the content of Mr. Boehner’s letter or his intentions to reach out to President Obama regarding tomorrow’s health care summit. His office informed me of his letter only after it had been sent to the President and released publicly. I am far more concerned with making real progress toward enacting comprehensive health care reform than with who received an invitation to tomorrow’s health care summit.”
Translation: I know full well that tomorrow's health care summit is a stupid political theater trick and I'm not sure I want to go on tv and be forced into saying publicly that the President is a tool.
Also, he didn't know about the letter beforehand? That doesn't ring true to me. But I could see someone remembering things that way after a little talkie with Rahm.