Wouldn’t know about the extended warranty. My first machine lasted about three years, but the warranty was up long before they offered the extended service. My second phat is probably going on about three years now, and I’d like to crack it open and dust it out.
I will note, however, that with both the regular and extended warranties, if you send in your console, they don’t actually fix it and send it back, they send you another refurbished unit, so you lose all your data on the HDD. You can re-download anything you’ve gotten from the PS Store, but you’ll lose your gamesaves. This was one of the nice things about fixing it myself. Even though it crapped out again, I was able to fix it again and transfer all the data to a new machine before trading in the old one (and since I kept it in good condition otherwise, and it was a soft-BC/4-USB model, and it was during a +%50 on hardware deal, I was able to get $150 credit out of it).
If it craps out, it’s not terribly hard to fix. You’ve got to take the whole thing apart and heat-gun the motherboard for a minute or two (check around online for specifics). However, after I did that, it only lasted about three weeks before it broke again. The second time I added some flux to the process (which I didn’t the first time), but right after I fixed it the second time I transferred the data to the new slim and traded in the old one, so I don’t know how long it lasted. (yeah, call me a dick for passing my problem on to someone else… hopefully it either held together, or crapped out again within the Gamestop warranty) A phat that I fixed for a friend (where I did use flux) at about the same time is still going, I believe.
To clean out the dust, you basically have to take the machine apart and just vacuum/blow out the dust buildup. They’re not that hard to take apart and reassemble (the phats, at least). You’ll need a torque-head (star-head) screwdriver (or small flathead) to get the main security screw out, but that’s the most specialized tool you’ll need. There are a couple ribbon cables with delicate connectors you have to be careful with. If you end up separating the heatsink from the cell and GPU, you’ll want to add some new thermal paste before putting it back together (Arctic Silver or equivalent… about $10). I did disassemble and clean out the old phat a couple years ago, and it lasted another year or so before it finally red-lighted.