Valve had made a number of rumblings about the PS3. They didn't like the architecture, which TBH as a PC based developers was understandable. From the point of view of Valve, and Epic and others, the PS3 was about as odd an architecture as you could imagine. The 360, despite having clear differences from PC, was a much easier target to transition to.
However, it's pretty clear that there has been a huge shift, starting way back with Halo on the Xbox, for exactly the types of games many of these PC developers make towards consoles. Epic and some embraced this early, as did Valve porting Half Life to Xbox pretty well.
However, it wasn't until the 360 that we saw a decisive shift, with Epic and others clearly more focused on the console than the PC.
Valve however, in no small part due to Steam, have remained PC centric and have used the 360 as an easy money machine, as their games can be delivered on the 360 easily and without much additional effort to Valve.
This I think was fair enough, and although maybe Gabe shouldn't have been quite so frank about his view of the PS3 architecture with respect to where Valve was, I think there decision to not invest in PS3 is pretty understandable.
Now however, the PS3 is really finding it's feet, and big FPS titles are selling a fair chunk of their total sales on that platform now. Like any sensible company Valve will be tracking the evolving market, and will of course revisit the issue of supporting PS3.
EA's port showed that, in principle, getting Source and Valve games wasn't necessarily going to be that hard. Remember EA were working with code that wasn't their own, yet they delivered what is actually a not bad port despite the bashing it took.
I think that where Valve to hire some PS3 specific engineers they could support PS3 without excessive cost, and it wouldn't surprise me if they did decide to start supporting PS3 as the return looks now like it could be worth the effort.
Looking at LFD 1&2 on 360, you have to figure there is a pretty big chunk of potential sales on PS3 that they could leverage. And given Valve operate on their own Source platform, once they did properly enable it on PS3 getting both past and future games on the PS3 would be easy.
Time will tell. But Valve are a smart company, and if the PS3 install base and support for FPS/Online titles is suitable, they will surely look to transition their assets over to that platform.
Personally, I'd love to see Valve on PS3 because on PSN, unlike Live, they could allow mods and access to user created content. They've remarked more than once they'd like 360 customers to get the access PC customers have to extended content - for example on PC I have way more campaigns/maps installed that ship with the game, for zero extra cost - and on PSN that would be possible as it is with UT3.