Yeah, it's a threat, whether you think so or not. I manage about 50 workstations, all Macs, and until recently we've been buying Microsoft Office 2008 for Mac with every new workstation. Since OpenOffice 3.1 came out, people are using it more and more, mainly because that's what they are using at home on Linux and Windows workstations. We no longer purchase Microsoft Office for Mac since OpenOffice is becoming the preferred office suite.
There's definitely a shift beginning to happen away from all things Microsoft when it comes to home computers. More and more people are being exposed to alternatives to Microsoft, simply through the products available from Apple, applications in the "cloud", set top appliances for home entertainment, alternative firmwares for things like wifi routers, and yes, even Linux distributions like Ubuntu that have steadily been improving the end-user experience over the past several years. Microsoft is not the end all be all company it once was, people are looking at alternatives, especially if the cost is significantly lower up front.
As much as I'd love to see everyone running Ubuntu and OpenOffice, I realize it's not going to happen overnight. But it is starting to happen in places I would have never expected just a couple years ago. This is the threat Microsoft perceives. If this shift gains momentum, it will begin to significantly impact their bottom line in a matter of years.
As for your experiences with OpenOffice, a couple of changes to Firefox would have it automatically opening .CSV files in a matter of seconds. Long load times? You are on a sub $300 notebook. Go purchase Microsoft Office 2007, or download a beta, and compare the two instead of blindly faulting OpenOffice for poor performance. It's probably the cheap machine at fault here.
In the end, you used TWO competing products to Microsoft Office, for free (minus your time). And you think Microsoft doesn't have anything to worry about? Have you purchased Microsoft Office for the netbook yet?