If someone in today's military receives a less than stellar (i.e. referred) evaluation report, is their career effectively over?

"Less than stellar" I would say is the norm for OERs, if you know how to read them. Most officers don't. Given that, than the answer is clearly, "no," as the majority of officers are promoted at every level up to LTC, that is promotion rates through LTC are above 50% of the in zone officers. If the question were reworded to say "adverse" rather than "less than stellar" then the answer would likely be "Yes" in this era of drawdowns, with lower than average promotion rates and officer separation boards. During an expansion like the Iraq surge, than even adverse OERs have little effect.

As an assignment officer at PERSCOM in the 90s, one of my tasks was to write a formal memo to any officer that received an adverse OER to ensure that officer was aware that the negative evaluation could/would have a negative impact on their career. Often these officers had no idea they had even received an adverse OER, which indicates they were either in denial, completely clueless, or their rating chain lacked the intestinal fortitude to look the officer in the eye and tell him/her that their substandard performance was reflected in a substandard evaluation.

My guess is that we are nearing the bottom of the drawdown as promotion rates are beginning to level off to normal peace time averages, which means we'll get back to an era where officers can in some cases survive even an adverse OER.

No, not necessarily.

My personal been there done that. I was a MAJ at INSCOM. I got a referred OER. Investigated ALL the inconsistencies and won. I retired 15 years later as an O-6. Because I gained a TS while at INSCOM I was called back post-9/11 and stayed 9+ years.

My feeling was you have nothing to loose by taking the gloves off and fighting for your life and career. I have also helped others fight referred OERs.

The answer is... depends.
If you are an E-7 and up and get a Marginal or 4/4 or below, you will face a QMP board. Not a death sentence, but not where you want to be.
For an officer, it matters where they are in relation to getting looked at for a board. If it immediately precedes one, you're in trouble. If you have a couple of rating periods in between, you have a good chance to recover.