Recent AMD drivers show that the company is holding back a Radeon HD 5900 series. As of yet, no details on these cards are available.
It's no surprise that AMD has a faster 5870 card planned. In the past, AMD also introduced a faster Radeon 4870, namely the 4890. Though specifications of a 5900 line-up have not yet been announced, it's likely that the core clock frequency will be increased. The first overclock results show that the 5870 can easily be clocked at 1 GHz, clearly leaving room for a faster model in the Radeon 5800 series.
It's unknown when the Radeon HD 5900 cards are to be released. It's possible that AMD is awaiting the performance of nVidia's new Fermi chip.
"ATI Radeon HD 5900 Series" = ati2mtag_Evergreen, PCI\VEN_1002&DEV_689C
"ATI Radeon HD 5900 Series " = ati2mtag_Evergreen, PCI\VEN_1002&DEV_689D