Here's what I did:

  1. updated firmware on N91
  2. reformatted hard disk
  3. installed LifeBlog

At this point everything was OK

I then installed Opera Mini (latest version, the built in web browser based on Apple's Web Kit doesn't work too well on my N91; it runs out of memory when you go to flickr for example! It's fine on Boris's E61 with flickr so this may be an N91 specific issue) and noticed over the next couple days several spontaneous reboots of the phone. Was this because of Opera Mini or is this just a coincidence? I have read (but can't dig up the link) of other people having the same problem

Needless to say I am not impressed with the stability of Series 60 v3. As an ex-developer I can however see that: i) it could very well be Opera Mini (but Java programs are sandboxed and shouldn't cause reboots right :-) ? ) ii) no software is perfect (although I didn't experience reboots like this with the N70) iii) this is Nokia's first phone with a hard disk which is much slower than flash memory which most Nokia phones use instead

My hope is that this will be fixed in future firmware updates and/or Opera Mini updates.