Well, of course, he didn't show up. You can thank the Wall Street Journal and other media outlets for creating the impression that he would.
At the time he announced his medical leave, he said he'd return to work at the end of June.
His appearance at the WWDC, cameo or otherwise, would overshadow all of the product announcements. His health would again be the story, not the new iPhones, Mac operating system and MacBook Pro notebooks. Jobs wouldn't want that. He's no dummy. He understands the media.
If Jobs is healthy, Apple likely will want to get that news out there quickly upon his return and not distract from some cool new product, like the rumored touch tablet. If he's not healthy, expect an orderly transition to make current Chief Operating Officer Tim Cook the new CEO. Cook is already running the show.
Jobs, a survivor of pancreatic cancer, could stay on as chairman and take a new title like "chief innovation officer." Apple's board of directors currently doesn’t have a chairman, but two co-lead directors in Bill Campbell and Arthur Levinson.