OMAP 3530 (a relative of our OMAP 3430) is designed to run at junction temperature of up to 90 degrees (the maximal temperature inside the chip).

The design is such, that if you keep the external surface of the chip at 70 degrees or lower, the maximum temperature will not exceed this limit.

have a look at page 124 here:
http://focus.ti.com/lit/ds/symlink/omap3530.pdf

I doubt if you exceed this 70 degrees.

There are four problems however
1. We don't have an accurate sensor to measure the heat sink temperature.
2. When the temperature is elevated, you need higher voltage to drive the chip, for me, ULV does not work when I am not in an air-conditioned room. When it's hot, you need more voltage (which generates more heat).
3. Smart reflex in the n900 is very buggy and crashes the device. If it were working, the device would have increased the voltage and limit the frequency by itself when it get's hot.
4. Nokia designed the cooling system to handle different thermal loads, it might not be equipped to handle what we put against it. I guess we are all aware of that.

What you can do is measure the battery temperature, it's a crude way of estimating the CPU heat sink temperature, but it's rather stable and simple to measure.

I assume there is up to 20 degree difference between the two.

So when it's above 45 degrees I change my device to the default kernel (limited at 600Mhz and normal voltage). I assume Nokia checked it @ high temperatures, I didn't.
When it's above 38 degrees, I run titan's ideal limited to 600Mhz to save battery power.
When it's below 38 degrees I run a custom extremely low voltage settings which hopefully increases my battery life.

This allows me to run icedove 24/7 and have a 8-12 hour battery life.