I found a nice Python script called ipcheck:

http://ipcheck.sourceforge.net/

It's a DyDNS client.

I think it's a natural fit with a n900 as IP changes can make it a little tricky to connect to the device. This way you get a stable domain name.

I copied it to my device and set up alarmd (using Alarmed) to run it once a minute. It only executes if something changes (or it's been 25 days with no changes), so it won't spam DyDNS with useless forced updates.