It's common knowledge that Stack Exchange has a reputation cap preventing users from earning more than 200 reputation points in a day.
Has the daily reputation limit always been a part of Stack Exchange? If not, when was the cap added?
This is pretty straightforward to find out.
Stack Overflow was publicly launched on September 15, 2008 (source: a quick google search).
The first version of the rep FAQ contains a mention of the point cap dated September 24, 2008 (source: look at the revision history).
Even earlier than that, there is a reference to it in beta from August 29, 2008.
So yes it's been around the whole time since the public launch, and at least partially through beta.
Another source of info for these types of things is the search, sorted by time. For example, as early as November, 2008 there were references to the cap (source: the last page of a quick search for "reputation cap" sorted by date).
...the current "200 reputation points per day through voting" limit
So it existed a very short time after Stack Overflow was publicly launched. As for the public beta before that, no idea...
There's also a feature request to get rid of the limit from 2009.
So the answer is either "always", or "very shortly after launch".