In my site, I have links to tutorials by site which look like this: http://www.mysite.com/site/www.othersite.com/ so when users click such a link, they get all the listed tutorials by www.othersite.com.

There is no real directory on my site called /www.othersite.com/ it's a virtual directory created and translated by php scripts and every thing is working fine so far but I was wondering: Could this cause me problems in the future (e.i. if I change the host, changed to IIS, new version of php....etc) Should I replace the dots with underscores or leave things like they are? What do you suggest?

"dots" won't be a problem

Thank you for your reply, you have saved me a lot of "not found" pages by Google if I had changed the dots into underscores

I was wondering because I have read somewhere that it's a problem for IIS to have ".com" in folder names because it thinks it is an executable (clever microsoft ) but I didn't know if that also would happen to virtual folders in urls or only actual ones. I'm not thinking to move to IIS (Apache is great) but I was wondering just in case.