Why is this a big deal? Well, HP is the largest technology company in the world, and sells the most computers in the world - so if it's considering adopting Android, even on a tiny part of its output, then we could be seeing the OS on a heck of a lot of machines in the future.
HP is also a major Microsoft partner when it comes to putting Windows on its PCs, and it has to pay the big M every time it installs Windows XP on a netbook. Google, on the other hand, doesn't charge a fee for Android, so HP could be looking to save some big bucks by switching.
On the other hand, the reason Windows XP is so popular is that most PC software runs on it - unlike Android, which has zero PC apps thus far. So don't count on the switch being made straight away.