One reason is that web designers nowadays like to use web fonts (usually in WOFF format), e.g. through Google Web fonts.

Previously, the only fonts that were able to be displayed on a site was those that the user had locally installed. Since e.g. Mac and Windows users not necessarily had the same fonts, designers instinctively always defined rules as

font-family: Arial, Helvetica, sans-serif;

where, if the first font wasn't found on the system, the browser would look for the second, and lastly a fallback "sans-serif" font.

Now, one can give a font URL as a CSS rule to get the browser to download a font, as such:

@import url(http://fonts.googleapis.com/css?family=Droid+Serif:400,700);

and then load the font for a specific element by e.g.:

font-family: 'Droid Serif',sans-serif;

This is very popular to be able to use custom fonts, but it also leads to the problem that no text is displayed until the resource has been loaded by the browser, which includes the download time, the font loading time and the render time. I expect that this is the artifact that you are experiencing.

As an example: one of my national newspapers, Dagens Nyheter, use web fonts for their headlines, but not their leads, so when that site is loaded I usually see the leads first, and half a second later all the blank spaces above are populated with headlines (this is true on Chrome and Opera, at least. Haven't tried others).

(Also, designers sprinkle JavaScript absolutely everywhere these days, so maybe someone is trying to do something clever with the text, which is why it is delayed. That would be very site specific, though: the general tendency for text to be delayed in these times is the web fonts issue described above, I believe.)

Addition

This answer became very upvoted, though I didn't go into much detail, or perhaps because of this. There have been many comments in the question thread, so I'll try to expand a bit (a lot of comments seem to have disappeared a short while after the topic was protected — some moderator probably manually cleaned them). Also, read the other answers in this thread as they all expand in their own ways.

The phenomenon is apparently known as "flash of unstyled content" in general, and "flash of unstyled text" in particular. Searching for "FOUC" and "FOUT" gives more info.

I can recommend web designer Paul Irish's post on FOUT in connection with web fonts.

What one can note is that different browsers handle this differently. I wrote above that I had tested Opera and Chrome, who both behaved similarly. All WebKit based ones (Chrome, Safari, etc.) choose to avoid FOUT by not rendering web font text with a fallback font during the web font loading period. Even if the web font is cached, there will be a render delay. There are a lot of comments in this question thread saying otherwise and that it is flat out wrong that cached fonts behave like this, but e.g. from the above link:

In what cases will you get a FOUT Will: Downloading and displaying a remote ttf/otf/woff

Will: Displaying a cached ttf/otf/woff

Will: Downloading and displaying a data-uri ttf/otf/woff

Will: Displaying a cached data-uri ttf/otf/woff

Will not: Displaying a font that is already installed and named in your traditional font stack

Will not: Displaying a font that is installed and named using the local() location

Since Chrome waits until the FOUT risk is gone before rendering, this gives a delay. To which extent the effect is visible (especially when loading from cache) seems to be dependent on among other things the amount of text that needs to be rendered and perhaps other factors, but caching does not completely remove the effect.

Irish also has some updates concerning browser behavior as of 2011–04–14 at the bottom of the post:

Firefox (as of FFb11 and FF4 Final) no longer has a FOUT! Wooohoo! http://bugzil.la/499292 Basically the text is invisible for 3 seconds, and then it brings back the fallback font. The webfont will probably load within those three seconds though… hopefully..

(as of FFb11 and FF4 Final) Wooohoo! http://bugzil.la/499292 Basically the text is invisible for 3 seconds, and then it brings back the fallback font. The webfont will probably load within those three seconds though… hopefully.. IE9 supports WOFF and TTF and OTF (though it requires an embedding bit set thing– mostly moot if you use WOFF). HOWEVER!!! IE9 has a FOUT. :(

:( Webkit has a patch waiting to land to show fallback text after 0.5 seconds. So same behavior as FF but 0.5s instead of 3s.

Addition: Blink has a bug registered for this too, but it seems a final consensus has not been reached regarding what to do with it - currently same implementation as WebKit.