Is the WAN IP of an ADSL modem likely to remain the same if it is not rebooted?
Do ISPs change the WAN IP of an ADSL connection while in session (ie. prior to reboot) ?
Likely, yes. Predictabily, no.
I don't know if they ever change "routinely", but they'll likely change at every line reset, and that'll happen during storms or power outages, just to name a few examples.
Bottom line is, don't rely on the IP to remain the same for any amount of time, but you'll probably be happy when using a service like dyndns.com for the occasional "get back home" connection.
It varies based on your ISP but you should be prepared for it to change routinely unless you've specifically paid for a service with a static IP.