DisplayPort is a new digital output format. It has support for higher resolutions than DVI, a connector that doesn't fall out all the time like HDMI, backwards compatibility with DVI and (in principle) VGA, better power management, daisy-chained displays, partial screen updates, cheaper transcievers, and probably more things the author is forgetting.
Support for DP in open drivers is a work in progress. This page attempts to document the current status.
For all drivers, the ability to drive a DisplayPort sink (monitor) using a DVI source is entirely a function of the feature set of the sink. No amount of change to the driver is going to make this work for you if it doesn't already.
Prior to DCE 3.0 (RV620, RV635, RS780), the GPU does not have DisplayPort connectivity natively, only over DVO. This does not work at all yet.
For DCE 3.0 (RV620, RV635, RS780) and later chips (RV710, RV730, RV770, etc.) using the UNIPHY transmitters, DisplayPort sources can drive DVI sinks using the radeon driver. Native DisplayPort connections work with xf86-video-ati from git master and with KMS.
The G40-series chips and later have native DisplayPort support. There is a branch of the intel driver that attempts DP setup. It's not clear whether this branch supports DP->DVI connections, but it certainly doesn't support DP->DP connections yet.
Prior to G40, DP support would be theoretically possible over SDVO. So far, no DP SDVO devices have been seen in the wild.
On G98, DP->DVI connections work out of the box with the nouveau driver. The nv driver has not been tested yet.
It's not clear that any earlier chips support DP.
S3 makes a DisplayPort card, but there's no open driver for it at all.
No other vendors (VIA, Matrox, etc) are known to make a DisplayPort product at this time.