If you've been into the 40k hobby for a few years, you'll no doubt have come across some people crying for a new codex for their race. GW updates their codices at their own pace, and expected sales are of course a big factor in deciding which race is next in line for an update.
I have played 40k for over 10 years now. I can field 1500 points or more of any army in the game, except for Orks and Imperial Guard. I have seen three updates to the Space Marine Codex, been overwhelmed by the awesomeness of plastic Necron Warriors, Tyranid Warriors and awaiting releases such as the Land Raider, Rhino MKII and the first Wave Serpent.
When I hear people talk about Tau needing a new codex, I get a little confused. This is because in my mind, the Tau are a pretty new race! Intrigued by this, I set out to make a timeline of GW's codex releases the last 10 years. Behold! (Props to MarcusZ, Turbo_t and jwolf_bols for helping me correct errors in the timeline)
The codices in black are the current ones per April 2010. The red ones are obsolete. (Note that I have not included the Blood Angels PDF release).
As we can plainly see, the "most deserving races" are Necrons, Dark Eldar and the Inquisition (Daemon- and Witch Hunters). Fortunately for you guys, the rumors have it that all these three races will se updates in the next two years. I am really looking forward to getting a glimpse of Jes Goodwin's take on the Dark Eldar!