Lately, my phone has been feeling like it is on fire. It gets so hot sometimes that I fear it's going to damage the device, so I take out the battery and place it somewhere cool to cool it off. But today I have taken the battery out 4 times already. And It is 7.00PM for me.
I have only used my phone to web browse (3G, no wifi) and place GSM calls.
I am not even playing flash videos or using apps with heavy graphics.

If I use the device too much, will a tendency for the battery getting too hot grow?
Also, is this an indicator that I need to get rid of the current battery and buy a new one?

send it back becaue it sounds like a defect.

Did you check CPU activity? That might be a reason. My old N95 8GB got hot while doing several things at the same time using a lot CPU power.

And you didnt overclock it?

Actually, I can remember one thing that heated up the N900: using the web over 3G on N900 and hooking it up to the Laptop got it pretty hot, but I dont think thats really a big thing. I guess the combination of charging the battery and intense use of the GPRS/3G was the reason for that.

Yep, it shouldn't do that. I'm not sure if a faulty battery can cause that (are you using the original battery?) but anyway you should contact the service.
Getting a new unit may be the only way to get rid of that problem.

Damn. How unfortunate.

It is not overclocked. Battery from original packaging.
How do I checked CPU activity? Htop app?

There are 2 apps for the status bar, dont know the name and Conky. There are maybe more apps, but the ones for the status bar are most convenient to check activity.

My N900 has never felt like that and I AM overclocking it. You don't happen to live in the Sahara or some other baking desert?

When i had my N900 i really sense when a call is placed (incoming/outgoing) the phone reallly increases temperature, that i felt my ear really hot and i got to change ear... The calls was like 1 hour (talking with gf :P) and was really strange... Now i got my N900 stolen and im gonna test with a new N900 that im gonna buy if the same happens

Yeah, not normal... I know there have been some reports that plugging the phone in to a computer via USB can cause it to cycle the CPU rather high. If you're doing that, stop it!

I also know there have been a couple apps that when installed can cause other apps or system components to run heavy CPU. Have you been seeing short battery life as well? If so, that may be whats getting you.

Check out the app manager for "Load Applet", which shows the CPU use in the top bar. If that shows activity, top or Conky can show you what the culprit process is. Based on that info we may be able to better diagnose whats going on. Good luck!

No kidding. Shoot, sometimes it feels hotter than the Sahara. 80 degree weather over here.

Dang, mine is really abnormal, huh?