Per-Henrik,
We just tried to diagnose this further, but weren't able to recreate the issue. When we enable SEF URLs in 1.7, the URL that we create for the Like button or Comment box is correct (usually the same as the current URL).
Can you let us know if you're running on a Linux or Windows server? Also, can you let us know what each of your options is set to in the SEO Settings area of Global Config (SEF URLS, Use URL Rewriting, etc). Finally, if you are using the "Use URL Rewriting" setting, have you renamed the htaccess.txt or webconfig.txt file (depending on your type of server)?
Thanks,
Alex