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I quit the game two years ago, and I'm just wondering if it's dead yet, or how close is it to dying.

That really depends on your local community - either there are enough players still playing to make it worthwhile, or they've all changed to one of the many alternatives.

Why would people who think 40k is dead be hanging about on a 40k forum anyway?

GW poops out new cool looking models every few weeks. Most collectors, painters and modellers go crazy about some of them every few months, so I guess sales might be up.
They have just released FAQ's for all the codexes and the rulebook, and have taken controll of their interenet communication. They now have a cool community site and a facebook site. Both attract a lot of attention and are rapidly replacing traditionally rumor sites.
The game gets an update in half a year nobody knows for sure what to expect there are wild guesses all over the internet.

Casual play is fun and rising in popularity at least in my area. The current rules for casual army building are great since you can now almost build any army you like. However semi casual play is a nightmare due to them abandoning the point cost system on the more competitive takes. I have no idea if tournaments are growing or declining.

Warhammer 40k, and GW in general, are doing better than they have in the past 5 years in nearly all aspects.

They're not where near dead, and probably won't be for a very long time.

Pretty far from dead. New Start Collecting boxes, new armies, and new supplements are keeping the game fun and alive. Plus a new edition is rumored. Why would you want it dead anyway?

Seems pretty good to me, though I have huge concerns about the next edition and general cartoony feel of some new releases. That said, I've had more and more fun with the game as time's gone on.

Well, 40k is more like an apocalypse game which allows one or more superheavies in an army. An extreme example is an army of 5 Imperial Knights which comes down to 1850 pts. Mass meltas can bring them down but in cc they stomp your army to death.
Missions are often maelstrom these days. These scenarios are tactically less flexible since you have to full objectives during each round. Armies successful in maelstrom missions often look rather strange.

They are not dead.
Competitive gaming is a shambling corpse held together by magical thinking.
The start collecting kits offer the first genuine deals in a long time.
The boxed games offer starter armies you can play "out of the box" which is a nice start for new players.
Those have managed to bring in interesting units like Sisters of Silence: GW is still putting out good models so at least that element of the hobby is consistently good (with occasional horror show model).
I agree that the health of the game for you depends on the local scene.
My only suggestion is that since 6th and now 7th edition, you may have to negotiate what to bring to the table with your opponent.
Since points values do not reflect the relative power level of armies it will take further work to get an "even" matchup.
It will be exceedingly difficult to get hold of all the rules so you will be very dependent on you opponent knowing their army's rules.
Maelstrom games tend to push armies to be tactically balanced due to the reliance on random objective conditions.

<edit> Seeing the note from above, it is very much Apocalypse based battles for army selection but they seem to have doubled-down on more skirmish rules that puts more emphasis on individual models.
It makes for long games so some patience may be required. Some people enjoy the added detail.

That is pretty much it as far as I know.
I hope it is in a condition you find workable.