Daring Fireball‘s John Gruber was writing a brief note about HP’s (HPQ) TouchPad tablet computer, unveiled Wednesday but not available before this summer, when he dropped the “3” bomb:

Summer feels like a long time away. If my theory is right, they’re not only going to be months behind the iPad 2, but if they slip until late summer, they might bump up against the release of the iPad 3.

The iPad 3? It was only that morning that the
Wall Street Journal
, citing the usual unnamed “people familiar with the matter,” reported that a thinner, lighter, faster iPad 2 with at least one camera had gone into production for release in April.

Apple (AAPL) rarely deviates from a 12-month update schedule for its major product lines (iPhones in summer, iPods in fall, MacBooks in the spring). What had Gruber, a blogger with unusually good sources within the company, been told?

Gruber quickly followed up with a longer post explaining that the iPad 3 remark was only a guess — not a thinly disguised “guess” that was actually based on something one of his Apple sources had told him — and that maybe the interim product would be more like an iPad 2.5 or an iPad 2 Pro.

But it was too late. The iPad 3 rumor spread through the blogosphere like a brush fire. By early evening, TechCrunch‘s MG Siegler had fueled the flames: He had been told by “a very good source” that Apple was assembling a “big fall surprise.” Moreover, he’d since heard that the new device is no iPad 2.5, but a full-fledged iPad 3.

“As of right now,” Seigler wrote, “the plan is apparently to release one iteration of the iPad in the next few weeks. And then blow the doors open with another new version in the fall.”

By Thursday morning, the story had moved to the top of Techmeme‘s news aggregator, with nearly three dozen follow-up stories trailing behind.

Not bad for a guess. Or was it a “guess”?

I have no inside information, but I’ve seen enough Apple rumors come and go to hold this one at arms length. It makes no sense for Apple to undercut the sales of the current iPad — never mind an iPad it hasn’t yet shipped — to be floating rumors about a new version two iPads from now.

If I had to put money on what next fall’s “big surprise” might be, I’d bet on some kind of iPod. Or maybe a new iteration of iTunes. Or some cloud-based service built around that North Carolina server farm that was supposed to come on line before the end of 2010 but has quietly fallen behind schedule.

Also on Fortune.com:

• Who can compete with iPad on price?
• One year later, 101 iPad wannabes
• Tim Cook disses iPad’s competitors
  

[Follow Philip Elmer-DeWitt on Twitter @philiped]