I received an email today, that contained this sentence:

In reply to:

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Rolling blackouts are a relatively new phenomenon that is unfortunately becoming more common.

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I know we've gone all around the plural/singular thing before, but I wanted to share this, and let people talk about it! It is, by the way, from a reputable company, and not the usual spam fare...

Suggestion for comment:

(The occurrence of) Rolling blackouts (is) a relatively new phenomenon that is unfortunately becoming more common.

I can't see anything special about the phrase. A similar example would be:
Public executions are still an accepted custom in some parts of the world.

The common denominator is probably the fact that the plural noun is derived from a verb in such cases.

It does sound funny. dxb's substitution is OK, but, how about:

Rolling blackouts are a relatively new phenomenon; A phenomenon that is unfortunately becoming more common.

Or, this?:

Rolling blackouts are a relatively new phenomenon that, unfortunately, are becoming more common.

a relatively new phenomenon that, unfortunately, are

I think it's clear that the that refers to phenomenon, which is singular.

is there a plural for phenomenon? red-faced edit: er, that would be phenomena, right?
could phenomenon act as a plural?
I do think that the author of this sentence probably spent time wondering about the plural are/singular is thang, however. I don't know why I think that, just a feeling...

Why not? So many people use phenomena as a singular.

It may be grammatically correct or not but it sounds wrong to have bothe the singular and plural in the same sentance. It would look neater to choose one and work the sentance around that choice.

Zed,
I agree.

it sounds wrong to have bothe the singular and plural in the same sentance

This is one of those problems that are confusing to many people

are you sayin' I'se confused?

I'se confused?

We're often confused.

We're often confused.

glad to know we isn't alone.