I was at the steam forum the other day and there was a topic about if hl1 mods will work on hl2.

The people from valves said they will make hl1 mods work on hl2.

Why:
well i can think of some reason why.
-cs 1.6, tfc steam, dmc steam,etc they are all hl1 mods and people play the games like crazy.
For the people who are going to buy hl2 at the shops without steam they are going to need to play the games online without steam so they are going to setup something like the way you open up the old halflife but specially for hl2.
-More people are going to buy the game in retailers becauses buying things over the internet isn't everyones favourite thing.
-People still love those halflife 1 mods (Like Public-enemy) and if u cant play them anymore well wat is the point of getting hl2. After u have finish the hl2 games wat are you going to do after that.that's why there mods.
i say on all the people who have hl1/steam only less then 1% play the normal half life. Everyone is into playing mods.
And Valves hammer editor for hl2 isn't going to be out for a while so who's going to make the mods.

So with the info on top you'll see that hl1 mods will be supported.