{Nope, not because he got fired or something.}
Valleywag leaked the story from an internal memo at Business 2.0:
Date: Thu, 8 Jun 2006 14:44:34 -0400
To: The Staff
Conversation: Om, Sweet Om
Subject: Om, Sweet OmTo: The staff
As many of you already know, Om Malik, after three years of writing about startups, Web 2.0, the GoogleNet and everything else, has decided to eat his own dog food. He’s accepted seed funding to turn his mega-popular blog, gigaom.com, into a real business.
When Om told me about his plan to start a company, I reacted predictably: I burst into tears, fell to the floor, clutched his ankles, and begged him not to go. This had nothing to do with Om having introduced me to the superior grilled lamb chops at Pakwan. I really like the dude.
Luckily, I was able to make him an offer he couldn’t refuse: Om will be staying on as a contributing editor, writing a monthly column for Business 2.0 that will start in September.
Watch this space for details of an upcoming pour celebrating Om’s, and our, good fortune.
JQ
This has forced OM to prematurely announce his transition. We can say that Om Malik has gone fulltime Problogging and his GigaOM Blog is only as big as Mike Arrington’s Techcrunch. 
Props to micro-pubs. 
[...] About four months ago, Om Malik announces that he’s gotten some VC funding for his GigaOm blog though he did not mention how much money he got to go fulltime. [...]