The thing is that I would like all of us to develop with whatever tools we feel more confortable with. I think it would be a loss that someone dind't publish their work just because of having to use a different tool than the one they already know. Or because of fear that someone starts asking about licenses of the software used.
Agreed, although it's nice for people who want to use/modify the design if they're able to open it, a design published in a proprietary CAD package is better than no design.
Anyone knows about examples of opensource hardware projects designed on such EDA packages?
Is there any policy of vendors such as A!tium on this kind of use?
Well, Eagle is not anymore open than those packages, only less professional, so I don't really see the issue. The only difference is that Eagle offers a free version (for simple designs and non-commercial use) that anyone can download, and an affordable commercial version, although some manufacturers might have free viewers (?).
I don't see vendors having any issue with this as long as you have a valid license, if commercial use is fine, open source shouldn't be an issue. They may be able to tell if it's a legal version from the files, though, I know Cadsoft (Eagle) is.
Would it be useful/usual to just publish schematic/pcb pdfs and gerber files?
Sure. I would also publish the proprietary files in case someone does have the package or has a way to convert. Unless the design is very complex, redoing the schematic should be fairly simple for anyone with the time and motivation. PCB is slightly more work, but if you have the footprints, tracing images isn't very much work.
Are there any GOOD schematic/pcb conversion tools out there?
Not that I know of, and no universal formats like DXF either.