They were available for the first time in 2001. You could buy them as an option to the EC model in Canada - I special ordered one of these and imported it into the u.s.; In the U.S. - the importer at the time, GGNA, bundled the ohlins and other options into a special edition model, "SE".
GasGas Corporate then imported directly into the u.s. without a independent rep as the importer - so from 2003 on - you could either buy ECs or DE (Dealer Edition) with the ohlins fork option.
The ohlins fork option, I believe, adds resale and marketability to the bike. The downside is that they are more difficult to service with a double bushing setup. The fork tube is thicker at the lower clamp and uses a different lower clamp than other models.
The marzocchi is a very good fork. i have a set of surplus cannondale zokes on the front of a xr280 trailbike I have; Les (of LTR suspension) has ohlins forks on his XR and twin chamber 50mm marzocchi's on a husky txc510 we share. For most model years, the zokes tend to be valved a little more harsh out of the box; After a little massaging by a competent suspension tuner, all of these forks work very, very well.
jeff