Contrary to some rumours flying around the net, Necrons aren't going to be scrapped, far from it. There will be a Codex: Necrons released after the Tau, so hold on until then for some brand new models, troop types and extra bits and pieces that you'll love. Lurking in the Special Character section of the Codex you will indeed find the C'tan. There will be a full range of plastics to support the new codex and the Necrons will have access to all sorts of funky new stuff and that includes a brand new heavy support choice.
Codex: Sisters of Battle is officially 'in the works', but as to when it'll be released...that's a bit more hazy, but we'll keep you posted.
Codex: Horus Heresy is planned, but is a long way off yet.
Codex: Tyranids is expected to be released in early 2001. However, Exocrenes and Malefactors will not be included in the Codex.
Apparently there may also be a supplement detailing close-quarter fighting in the 40k Universe featuring rules for destroying buildings etc. However, this may be linked to a re-release of Necromunda (see previous rumours)...