It doesnt matter if your PS3 is vertical, horizontal, upside-down, or sideways. As a victim of this YLOD issue myself with my launch system, and after seeing all the reports on many different forums of the many, many, others who've had it happen, it's my opinion that Sony has/had a manufacturing problem with the old PS3s and eventually most of them will YLOD far before the system should have "normally" quit working. The worst part is Sony's continuing arrogance regarding the whole thing, pretending its not even a real problem. Many people like myself end up with a YLOD system when we treat our systems like gold, clean them regularly, give them plenty of space, etc... It's NOT our fault these things are dying.
In my small, local circle of PS3 gamers, there's 5 of us, all living in different geographical areas. Out of the 5, 3 of us have had YLOD on our PS3 launch units right around 2.5 year mark. Not quite as "isolated" of an issue that Sony wants us to believe.
Since someone asked, reflowing is the process of taking the PS3 apart and heating the solder around the GPU and CPU (with a heat gun) to a point where it liquifies again, and then rehardens. You then reassemble your PS3, and in alot of cases it fixes YLOD since the process creates a new "seal" with the chips to the board. The problem is its only temporary. If you get it to work, you're only looking at a few months, tops, before it happens again.
Since Sony refuses to even acknowledge that there's a potential issue with their older systems and YLOD, or even ATTEMPT to look into it, i have no other choice but to believe that 2 years or so from now all of our Slims will be having the same issue.
I love my Slim to death, and the PS3 and its games in general, but man.....Sony needs to deal with this YLOD BS now!!!