Update : it seems many people are experiencing flickering problem with the mac mini (varies from monitor vendor, Samsung’s are reported to work bad with the mini). A workaround with previous minis (without HDMI) was to connect to the monitor using VGA (through mini DVI to VGA adapter)… Replacing the mac mini/motherboard many times didn’t work for these people, so they are implying the problem comes from Apple’s driver software (and thus could/should be fixed in a later update) :

http://forums.macrumors.com/showthread.php?t=989930
http://www.ehmac.ca/mac-ipod-help-troubleshooting/80230-flickering-mac-mini-lcd.html

The Samsung 206BW doesn’t seem to allow refresh rate above 60hz at 1680×1050, however some have had good result (with other monitors) using 58 or 57hz (using a third party utility). I tested lower resolutions through Apple’s monitors preferences pane, and some provide additional refresh rates (70 and/or 75hz) : for most, switching the refresh rate higher didn’t fix the flickering issue. Moreover, the 1024×768 resolution didn’t experience flickering, even at 60hz, while the “1024×768 (stretched)” (in fact both modes display stretched image) did flicker whatever refresh rate !
(submitted at 08-15-10, 15:00 by cjed)

As some users had less flickering connecting the monitor through VGA, I bought the Apple mini display port to VGA adapter, and here are the results :
-the flickering seems gone (at least very slight), particularly on gray tones where the problem was huge.
-however the image now has a pink tint (on white), and colors (white) are oversaturated (no mean to fix this through ColorSynch settings nor monitor's settings).
-the text and controls borders are less sharp.

Then it isn't really usable, and I'm stick with DVI and flickering (that at least mainly occurs on gray colors, hopefully not the most used).
Apple have to provide an update to GeForce 320M chipset driver, as it is likely to be a software problem.
(submitted at 08-17-10, 22:51 by cjed)