You cannot read too much into stories of early investment. Despite the insanities of the first internet bubble, venture capitalists still often show questionable judgment in choosing which endeavors to fund.
Regardless, seeing significant investment in a Rails oriented startup can be interpreted as a sign of the framework being taken seriously as a business tool, as opposed to an open source toy. I do not know if FiveRuns is the first such company to receive this kind of investment, but the fact that they make systems management software for Rails also points to a real market for supported and managed applications and applications development.
I will admit to be a little confused after reading of CDBaby’s travails with their Rails re-write. Perhaps this interest from the business community will see an increased focus in scaling the framework. It’s not a guarantee and this outside interest is likely to generate some friction with the core framework maintainers as they will want different priorities in future development almost as a certainty.