I was just given a Nokia AC-3U charger and noticed that it has the same 5v output that the defacto charger that came with my N810, but it has like 350ma output vs 890ma.

Is it safe to use this as a backup charger?

Given the lower amperage it would at best charge the N810 much slower and might not achieve the same level of charge as the original equipment charger.

I use it too. It charges the battery fully, but takes twice the time.

350mA output is useful for when the battery's internal resistance has become so high N810 would otherwise with original charger consider it too unsafe to charge and reboots..

But once you get to that point you probably want to get a new battery anyway