From the unfounded-but-exciting-rumour department, Brighthand reports that an anonymous tip has been received, indicating that PalmOne may offer OS6 (aka Cobalt) as a pay-for upgrade on some higher-end OS5 models.
Still, this situation may yet change. An anonymous source has posted on the 1src Forums an unconfirmed report that palmOne plans to release Palm OS Cobalt upgrades for two of its high-end models, the Tungsten T3 and the Tungsten C.
Sounds totally unfounded, but the source of the rumour has a small amount of credibility:
This source appears to know a handheld developer with access to pre-release versions of palmOne handhelds. This is the same person who leaked the new details on the Tungsten T5 and Tungsten E2.