Careful about paying a lawyer up front, though, because it's apparently very hard to make a detrimental reliance claim about a rescinded at-will job offer; it appears to be impossible in several states (at least Florida, Louisiana) and very hard elsewhere.

In the US, most jobs are at-will (you can be fired for no reason, and you're not supposed to rely on your employer maintaining your job); a Rails dev job certainly is.

That said: Patrick is almost certainly right that your company will be happy to keep you on. It'd be almost irrational not to.