I noticed O'Reilly was selling them (the computer book publisher). Do they or something else own the copywrite on these shirts? I want to create my own I'm Blogging This t-shirt.
Do you mean, does someone own the trademarked phrase "I'm Blogging This"? (Kinda like Donald Trump's "You're Fired" phrase?)
As momrocks pointed out, it would be trademark. Copyright cannot be extended to words, names, or phrases. Copywrite is something totally different (text written for ads, brochures, etc...) and as a term has nothing to do with intellectual property. It does not appear that it is a registered trademark. However, trademarks can exist without registration. These are called commonlaw trademarks and are developed through usage. Since you saw it on a shirt that O'Reilly is selling, it is safe to assume that it would have commonlaw trademark status (at least in for clothing). That's not to say you could never use the phrase or that you couldn't use it for a different industry, you just shouldn't make shirts with that on there and try to sell them.