Question: In episode one, when Baltar is seduced by what turns out to be a Cylon in the form of a human, the Cylon's spine glows red and shines through her skin. Was this just a special effect for the benefit of the TV audience? Do any or all of the other human looking Cylons in the 2004 series suffer from this give away indicator?

Answer: 
I don't believe this is ever specifically given a reason. However there are two occurrences as far as I can recall:

The first is with Baltar and number 6, who are apparently deeply in love.

The next is with Boomer and Helo, who again are both apparently deeply in love.

Since the human Cylons were designed to not be detectable to humans, it seems unlikely that it is in their design for their spines to glow during sex. Potentially the fact that these Cylons were expressing love to *humans* cause this abnormal appearance to manifest itself. 

The most likely explanation is probably as you put it "just a special effect", which is backed up by the [wiki](http://en.battlestarwiki.org/wiki/Miniseries,_Analysis), though without reference:

> 
> Note: Comments from **members of the production crew have since suggested that the only reason the glowing spine was included was that it "looked cool" at the time**, and in retrospect, may have been a mistake. According to the novelization, the spines glow in the infrared spectrum, which would mean it would require special optical equipment for it to become visual.
> 
> 
>
According to the [official novelisation](http://en.battlestarwiki.org/wiki/Battlestar_Galactica_(2005_Novel)), the glow was into various spectra that couldn't be seen with the naked eye. There's no special indication *why* her spine was glowing, other than it grew with her "sexual fervor", suggesting a physiological response akin to orgasm.

> 
> As their lovemaking mounted toward a climax, the wall behind her was
> warmed slightly by a peculiar, nearly invisible light. The doctor
> never saw it—and wouldn’t have, even if he had been less distracted.
> The light was mostly infrared, with just a hint of gamma radiation. If
> his eyes could have seen it, they would have seen the glow of fiery
> embers, the glow of heating coils. It was a soft glow, but growing in
> intensity, growing with the woman’s sexual fervor. Indeed, it came
> from, and illuminated, the spine of the gorgeous, naked being who was
> rocking and bobbing as she made love to Gaius Baltar.
> 
> 
> [Battlestar Galactica: The Series](http://en.battlestarwiki.org/wiki/Battlestar_Galactica_(2005_Novel))
> 
> 
> 

The *canonicity* of this event was addressed by Showrunner Ron Moore in an interview to coincide with the series finale. In short, the "*glowing spine*" thing wasn't retconned, it just wasn't something that came up again (much).

> 
> **Q:** *The glowing red spine (during sex) in the first season. Was it just
> abandoned?*
> 
> 
> **Ron Moore:** ***It wasn't really abandoned;** we just didn't do a lot more Cylon sex
> scenes.*
> 
> 
> ['Battlestar Galactica' finale: interview](http://articles.latimes.com/2009/mar/20/entertainment/et-burning-questions20)
> 
> 
>