Sprint Premier customers are being treated to a rather tantalizing account alert tonight: "Visit us tomorrow for an exciting announcement about a groundbreaking new device." No other details provided, but if we had to make an educated guess, we'd say all signs are pointing towards HTC's Supersonic, the long-rumoredWiMAX smartphone with a 4.3-inch screen and Android 2.1 with Sense UI. A 4G device would certainly fit the billing as "groundbreaking," and we've already heard of plans to unveil the phone at CTIA, which officially kicks off tomorrow. A web chat transcript found on the Sprint Community seems to corroborate, but we can't at this point verify the validity of the screen capture. You can view the entirety of the purported web chat after the break. If we do wake up tomorrow to a shiny, large-screened WiMAX phone, care to take bets on what Sprint CEO Dan Hesse is gonna talk up in his Wednesday keynote?