last Switched On discussed why HP could have more success licensing webOS than Palm or PalmSource ever did with with Palm OS. To put it in the context of a more modern conflict, HP's handsets could be the equivalent of a Google Nexus devices (but selling better in HP's ideal), competing with phones from other Android stakeholders. Even the Nexus phones, however, are ultimately produced by existing licensees such as HTC and Samsung.
WebOS as a licensed operating system would likely compete most directly with Windows Phone 7, an OS that offers licensees and consumers some choice but preserves a consistent user experience -- particularly as it is trying to court developers. Unlike Windows Phone 7, though, webOS is rapidly being expanded to new form factors, with the TouchPad serving as the first tangible proof.
HP has said that it's most interested licensing to companies that wouldn't compete with it in its core markets. For now, let's count out HP's major PC competitors Acer, Dell (which once may have tried to build its own webOS-like platform when it acquired Zing), Lenovo and Toshiba. However, many companies that could help develop meaningful (in terms of absolute volume but also as a relevant development platform) scale for webOS in at least the US market offer, at minimum, handsets. A handset licensee could imbue webOS phones with features such as a 4.3-inch display that HP has shied away from, but which has been present in many successful smartphones.