There are no confirmations from Valve or Sierra, but a blurb on Next-Generation Online quotes "insiders at Sega" as saying an "Internet capable version of Sierra’s Half Life will appear on Dreamcast in the near future." Word is Sierra could not be reached for comment on the rumor at the time the story went up. DREAMCAST.IGN.COM is even more confident, calling the word from Sega a confirmation:

At Sega's pre-TGS meeting in Japan, it was announced that Half Life would be seeing release for the Dreamcast in late 2000. That's a long time from now, but the waiting period is apparently due to Sega's desire to equip the title with full online multiplayer capability.