If it is not, how do I enable it?
Your question is misleading - its got very little to do with Ubuntu. You don't say what webserver you are using - but its probably Apache. But even that is not much help - if-modified-since is a request header sent by a browser - do you mean that your browser (what browser?) running on Ubuntu doesn't send the header? Or that your webserver doesn't respond as expected? The browser will only make a conditional request (using if-modified-since) for content served with HTTP/1.1 caching information. Is the webserver issuing relevant caching instructions?
Even if you'd provided an accurate description of your problem, its worth bearing in mind that conditional requests often undermine performance rather than improving it. About the only exception to this is where the majority of your content is unusually large static files (e.g. audio and video files).
@JMan: Appears you're running Ubuntu (Lucid) and using Apache2 and PHP.
Also, this appears to be related to PHP, not Apache -- try Google "if-modified-since apache default php"
For related docs Google "Caching Guide - Apache HTTP Server" and select the version of Apache you are using.