I have been doing some more stability testing of running 'Mapper' with tracking enabled and other applications to see what can cause the N900 to go into a death spiral where your only option is to pull out the battery to restart.

Here is what I have found to date:
Mapper with tracking by itself is fine for at least several hours.

Mapper with tracking and MfE is fine for at least several hours.

Mapper with tracking, MfE and browser can cause problems. Heavy sites can make the response time of the N900 drop noticeably. Additional applications or flash sites can really slow the N900 down and I suspect this is when the N900 is likely to go into the death spiral.