Replicant

Well, I think modem isolation won't be a problem, since it was designed with exactly that in mind. As for freedom, there's still be the usual non-free firmware for Wi-Fi, non-free modem operating system, non-free bootrom, etc.
Still, the bootloader will be fully free and the device will undoubtedly allow running a free system such as Replicant easily. Only graphics acceleration and acceleration media decoding should be an issue.

Note that the GPS will come from the modem, which might be a problem regarding privacy/security. In any case, Joerg's (the Neo900 EE) stand is that GSM is sufficient to provide the operator with your location with a precision equal to GPS, so the fact that the GPS is attached to the modem doesn't matter as it doesn't provide the operator with more information than it already had.

I do have the intent of getting a Neo900 and porting Replicant to it.

Then what advantages Neo900 will have over S3 w. Replicant? Price difference is huge (~250$ vs ~1000$) and S3 actually has better hardware.

Well, the neo900 was made by a small community with free software in mind, which is already a good sign!

The major differences are first about privacy/security: the neo900 has features to probe what the modem is doing, something any other device is lacking. It also allows running a free bootloader, while the Galaxy S 3 (I9300) has a proprietary one that cannot be changed. It will also have more hardware features working in the free world, such as GPS, something the Galaxy S 3 (I9300) is lacking. You can also expect that the neo900 will run other operating systems such as Debian, QtMoko, perhaps Firefox OS, etc.

To be honest, I barely see the "everyday phone" running Debian with desktop WM. I bet community behind the Neo900 is really looking forward to Replicant.

From what I could gather, many neo900 enthusiasts are looking forward to it running the same Debian-based Maemo system that was originally on the n900 (including many proprietary parts). But of course, it'd also be useful to have Replicant available for it!

+1 for supporting the Neo900 device. That would be awesome. :-)