i had only overheard a brief rumor about possibly hitachi releasing one sometime (it mightve been a different company that was mentioned, not sure)
but as far as the larger platter capacity, and faster rpms... thats actually a main physical limitation right now, and the reason raptors and other 2.5" scsi hdds are stuck at such small capacities (74GB platters, compared to 250GB 7200 platters), with older technology even (LMR, known for reliability instead of brand new technology, such as pmr)... ...first you have the smaller 2.5" platter which allows for the higher rpms (compared to 3.5" 7200s)... the consequence of higher rpms is more heat and noise is generated per platter too (which is a complaint of raptors as well, which might be why raptors only have 2 platters)... (though the 300GB hitachi runs at 15Krpm... i can only assume it has 4 platters (or possibly 2 larger platters even...)... if it has 4 platters, i can imagine that that is one very hot hdd
the bright side in all this though, is that IF another company releases a 10k rpm sata hdd, they might very well be using something like perpendicular magnetic recording, like your seagate has... the total capacity might not be as high as your current hdd (due to heat and vibrations and such per platter, and thats not good at all for the hdd), but you could pretty much guarantee it will be faster
(thats if its possible to even incorporate PMR on a higher rpm hdd, no reason why it shouldnt be, i wouldnt think, maybe, though it could just simply be a physical impossibility right now, vibrations most likely)