Endagadet and Homemediachoice recently teamed up to give reviews of the PS3slim. 

While i'm not going to post the entire thing, I think that the heat issues are best explained by geeks that are better than me:

"In operation

When you power up the console, the first thing you need to look for is a system update.  My console found v2.80 and within a few minutes this had downloaded and installed.  V.3, which brings some extra functionality, has yet to make an appearance at the time of the writing. 

The original PS3 was significantly quieter than its Microsoft rival when it came to operational noise.  While thte Xbox 360 sounds like a hyperactive vacuum cleaner, the PS3 maintained a dignified hum. 

This model, at least initiall, is quieter still.  However, heat builds up quickly during use and the fan noise  accelerates accordingly.  Midway through X0Men Origins: Wolverine, I noted that the console was disgorging a lot of heat through its rear vents, and at that point, in terms of fan noise it did not seem to be an improvement on what's gone before.  The huge 95mm, 17-blade fan beneath the hood clarly has its work cut out...."

Realistically speaking, Sony has probably put these consoles through enough tests to know whether or not they will have heat issues, and to what degree. If this isnt really a factor then sony sees an advantage in making a console that proably costs $50-60 less to produce than the previous model. So, while we may see a few more reports of heat related failure, I doubt that it will be much more than it was before. Remember, Sony is an electronics HW company, I don't think that they will tarnish their reputation as a company that produces quality products.

All consoles heat up....how is heating up a issue??

Why do they think the FAT PS3s are so fat and all the old skinny xbox360s got RROD. The RROD is pretty much gone in the new 360s and the PS3 can now be the size of a 360. The issue was the amount of heat in the space provided and how easily it is expelled from the machine. If no heat was coming out, then I would worry.

I think sony took some vents out for the new slim model, pic414

Right but i feel the article is fear mongering. They are taking the overheating problems the 360 has and the facts that the PS3 slim generates heat (like all electronics) and has a "loud" fan that turns on to dissipate it to try and make it seem like the new PS3 will definitely have the same issues. I'm not saying for sure that it won't but Sony has a reasonably good hardware record. Also saying it blows out hot air and is louder is rather subjective. If they had facts like temperature comparisons and decibel levels over time then there might be able to be some discussion about what problems could be caused.

Oh please, that is nonsense. My launch 60gb PS3 gets ridiculously loud (to the point where my friends have said that it "sounds like a jet plane about to take off) and spews out sooo much heat that it is basically a second heater, so until the slim's fan starts to get loud (and I'm sure it will with that huge fan), I wouldn't worry about it. Anybody remember that article about running a PS3 on a sauna? Somebody should do that test with the slim.

i'm sure microsoft put their system through "rigorous testing" before relase and wahey bang RROD i am convinced this PS3 slim will be at least a contender for noise level winner of the generation

Personally, while I think it was a bit odd of Sony to have the HDD so close to the BRD in the slim, it's no odder than them having the HDD near the power supply in the phat.

I'll trust Sony on this one.
After all, it didn't say the thing caught fire or failed or anything