I don't know of any specific log file where network status is stored, unfortunately there isn't a whole lot of access to the baseband stuff, though you could install netmon (apt-get install netmon) and check the cell info screen to see if anything stands out on there.

Edit: Just wondering, do you know which firmware version you have? There are some operator and country specific versions floating around, not that this really matters a whole lot since I've never heard of an N900 being locked to a specific carrier so any sim should work in any N900.