GW are advancing the metaplot through novels, the new codexes (and new armies thereof) and the worldwide campaigns.
This is how it stands.
Hive fleet Behemoth is busy munching the eastern fringe. The Tau are holding them back, the imperium aint so lucky. Another hivefleet (cant remember the name) is advancing up the galactic spiral, aiming towards the homesystem (earth).
The Necrontyr have awoken from their 60 million slumber, and boy are they pissed. The book "Nightbringer" detailed the awakening of one of their gods (the C'tan Nightbringer). Another one is on mars, possibly pretending to be the machine god. Yet another one (the C'tan Deciever) started the Gothic war (as in BFG) in order to remove the Blackstone fortresses which were eldar weapons of war designed to kill the C'tan. Most were destroyed; the remainder were held by Abbadon. This will probably be another world wide campaign at some point (2005 is my guess). The Necrons are clashing with everyone, attempting to land on mars, and hunting down human pariahs, warp resistant abberations, for use as soldiers against the eldar.
The Tau have turned up. Not much happening on that front as yet.
The orks have united to a great deal under Ghazghull Mak Urak Thrakka. The war for armageddon continues : black library just released a book detailing all 3 wars for it and a lot of background info.
On the chaos front, well. The Inquisitor Czevak, the only man outside the eldar craftworlds with knowledge of how to get to the black library (the eldar library containing information on the specific nature of the chaos gods), has died - commited suicide i think. This failed in its purpose - to prevent Ahriman to get the location of the BL. He managed to get it from the inquisitions network, following the crumbs in the data vaults (he got a imperial savant to do it, amusingly enough). Thats bad - he now has the potential to become a veritable god.
Otherwise, all the legions have united under Abbadons banner - the 13th black crusade is due to begin. The exception are the iron warriors who have, for some reason, declined to join in. We will probably learn why later. In return for not fighting, they took a citidel on Hydra Cordis, which contained thousands of geneseeds - enough to create a whole new legion of chaos marines, or to bolster the existing ones massively. This was described in "Storm of Iron". Bad news.
There is some interesting stuff on the Abbadon front - he knows about the C'tan on mars, and wants its power. He will also likely achieve daemonhood during the campaign, or as part of the metaplot afterwards.
The 2003 summer worldwide campaign "Eye of terror" will include the following metaplot advancements :
The entire Cadian command structure will be assassinated by abbadon prior to a massive invasion.
The eldar and the darkeldar will fight a war in the webway - the eldar have decided they cannot allow abbadon to win, the dark eldar think that allowing him to win would be a really cute thing to do. The ulthwe crathworld will use the webways to act at the precise moment of necessity, aiming to turn the balance against abbadon.
Thats all i can remember right now. Essentially, the Imperium is screwed.