There are no plans yet to ever integrate Servo directly into Firefox. At the very least Servo will certainly exist as a standalone browser engine (and unlike Gecko, Servo is designed to be embeddable, so it will be an actual alternative to Webkit in that space).

The biggest impetus for Servo at the moment is that it's researching the biggest wins to be gained from concurrency and parallelism in browser engines, thereby guiding efforts to tack such things onto Gecko.

Servo aims to be "dogfoodable" by the end of the year. It's still an experiment, really, so you won't be able to get anybody to commit to any plans further than that. The design is significantly different from Gecko that I suspect you'd never be able to wedge it into the existing Firefox, but it wouldn't be entirely implausible if a usable browser called Firefox used Servo within 5 years.

As well as the other replies to this, note there's no plan to implement a lot of the non-standard parts of Gecko that Firefox (and most extensions) rely upon, such as XUL and XBL. Servo is concerned with the web (at least for now) — not replicating Gecko's feature set.

As far as I know there are no short- or even mid-term plans to integrate Servo into Firefox. It's still a research project first. Rust for example doesn't even have a stable API yet.

It may never happen--It's a research engine, not necessarily something that's going to replace Gecko anytime soon.