Update:
First, thanks gents -- you guys were 100% right regarding the navi updates not affecting anything.
Second, apparently the problem I've had -- the nav screen going blank occasionally as well as intermittent poor ipod connectivity -- is not merely fixed with a software update, but with replacement of the whole nav touch screen!
See also here:
Factory Nav Ipod Problem
I'll post details when this is done as well as scan and post a pdf of the relevant TSB -- it's not 370Z specific, and I don't think it's been uploaded here previously.