HP is a minor player in mobile computing at the moment, but they're not ready to give it up. The Palm acquisition puts them seriously back into that game. Up to now, HP has been a pretty faithful Microsoft partner, even when Microsoft broke a promise to HP during the Vista Capable logo debacle. HP's current iPaq phones use Windows Mobile 6, so it would seem like Microsoft has the inside track to HP when Windows Phone 7 comes out. Then again, not so fast.
Purchasing Palm gives HP several other options for a new generation of phones. It's possible that they could use Palm's current OS on their phones, but to me that seems the least likely outcome. Instead, HP seems more likely to use either Google's Android or Microsoft's Windows Phone 7 as their platform. HP and Microsoft have a long-standing patent cross-licensing agreement. Even if HP builds an Android phone, it wouldn't be subject to the kind of financial shakedown that HTC just suffered at the hands of Microsoft. That puts HP in a good position, at least IP-wise, to play in the mobile device market. But it also helps Microsoft, because that same agreement makes Palm patents fair game for Windows phones.
It's possible that HP could even go on the offensive like Microsoft is doing, making calls to cell-phone makers and demanding they take licenses for relevant patents that HP now holds by way of the Palm play. Companies like HTC have come out of nowhere over a short few years, and can't defend themselves by cross-licensing patent portfolios with Microsoft or Apple the way a company like Nokia, Toshiba, or Motorola might do. Seems like that's going to be costly for them.