How to use Screen Sharing if I don't know the remote Mac's IP address
Ah! Yeah... your options at this point are limited. The ISP might help you as it's their DHCP that assigned the address; they'd be the only ones that would know that. 

This has happened to me, and as my ISP is comcast, there is no help. 

In the future :), what you need is "Dynamic DNS"; available free from several sources. I use FreeDNS. It's kind of funky, but seems to work reliably. 

The other alternative (to dynamic DNS) is a "roll your own" approach that would require some script-writing on your part. If you're up for that, let me know &amp; we'll go from there.