Valve's Jenni Gant sent around this word that Half-Life owners who want to try the Uplink demo can now access it from the mod browser:

If you've got Half-Life installed and haven't downloaded the Uplink demo yet, this message is for you...

The Half-Life Uplink demo (which includes a mini-mission that's not in the original game) can now be downloaded using the mod browser that we integrated with our HL v.1.0.0.9 update. This version of Uplink is only 10.4 MB (versus the 50 MB download for the original "non-mod" version) because it uses files you already have installed with the full game.

To download Uplink:
1) Launch Half-Life.
2) Click on 'Custom Game'.
3) Click on 'Refresh list'.
4) Select the Half-Life Uplink demo from the list.
5) Click on 'Install'.

Have fun...
Jenni Gant