HTC looks to be bringing forward a novel design for it's sliding mechanism with a new, recently granted patent. The new mechanism uses springs inside a self-contained "sliding rod package" made of self-lubricating plastic. The springs prevent unintentional movement of the slider and provide automatic assistance sliding the device open and closed.
Two stainless steel low friction rods provide the guides on which the device slides open and shut. The patent specifically mentions the slider is intended for use with sidekick-esque devices, which is interesting with HTC's more recent devices like the HTC 7 Pro and HTC Arrive utilizing a slide-and-tilt mechanism, which does not seem compatible with the sliding rod design detailed here.
In addition to usage on a sidekick-esque device the patent mentions "a sliding cover with a big span length", which could allude to a future larger screen device (4.3'+) with attached QWERTY keyboard. With Samsung and T-Mobile bringing back the Sidekick line, perhaps this indicates HTC is preparing a challenger to the popular lineup.