I've read the entry in the MM for the Half-Celestial template. It applies to a non-evil creature and the creature becomes "always good". What does this phrase mean? What happens with a half-celestial character that tends to neutrality or commits evil deeds?
"Always" isn't precisely the right term
A creature that is "always" a certain alignment is only that alignment about 95% of the time, with the other 5% being composed of various exceptions such as those created by spells, an inclination to a different alignment, corruption, redemption, or just plain cussedness.
Alignment is contradictory
Though many sources (the Player's Handbook, Book of Exalted Deeds, Book of Vile Darkness, Heroes of Horror, Champions of Valor and Champions of Ruin especially) talk about alignment, they don't particularly get along with each other. Alignment is a contradictory mess full of double-standards, exceptions, confusing 'hard rules', and just plain lack of moral thought. This makes it hard to talk about in 'always' cases, especially ones like Half-Celestial that were published early on.
However, free will is a common trend
Think of Half-Celestials as having an innate instinct for goodness - they understand about mercy, empathy, compassion, love, and forgiveness the way normal people understand about breathing. As free-willed beings, they don't have to listen to these instincts - but they do have to live with them. Evil is not comfortable for such beings. Perhaps some turn to it out of a perverse thrill, the way masochists get attached to pain, but outside of those exceptional few evil is difficult and uncomfortable for beings whose essence is at least partially composed of elemental goodness the way a fire elemental is made of fire. No, there's nothing stopping a fire elemental from taking up, say, water skiing but it's certainly not the best hobby in the world for it.
There are examples of 'always' beings outside of those alignments
Fallen celestials, lawful slaadi, chaotic modrons and risen fiends exist in the 3.5 universe, often as published NPCs. The alignment rules state that if a being acts in accordance with a new alignment, they take up that alignment. From a mechanical perspective, if a half-celestial does consistent evil, then it is evil-aligned.
According to the Monster Manual v3.5, p.305, "always good" means the creature is only found without that alignment in rare cases:
Sean K Reynolds' 2003 sourcebook Anger of Angels presents some interesting templates for celestials who are not good-aligned. This usually occurs when an angel is cast out for accidentally or intentionally committing evil acts or violating the deity's rules. A deity may also create evil angels to commit evil acts for the greater good.
According to the Anger of Angels falling from grace rules:
As I understand it, the 'always X alignment' entry means unnamed NPCs will be of that alignment. Rare exceptions can and do occur, with the exceptions representing the truly unusual and/or exceptional individuals among a race. This conformity might represent a powerful genetic tendency or a society so strict it virtually precludes the possibility of developing another point of view.
For certain beings, it's possible this is enforced in a supernatural fashion, like an ex-paladin losing his magical class benefits. For others, the exceptions could simply represent those who've become outcasts from their native society, those disillusioned with what they once stood for, those who've fallen afoul of strange magic (the helm of opposite alignment comes to mind!), or those traumatized to the point they're driven mad (or driven sane, for some races!).
In short, the 'always' term applies to every member of a particular race except for any character so unique they have their own backstory to explain why they're not that particular alignment. This almost always results in what we would consider a 'named character,' and those are 'always' the exception to the rule.
Of course, for something like a half-celestial or half-fiend, they are considered to be a member of the relevant alignment for such purposes as detect evil/good or the extra damage from a holy/unholy weapon, regardless of what their actual alignment is. In fact, it is entirely possible for such a character to count as lawful, chaotic, evil, and good at the same time!
It almost goes without saying that any PC is going to be unique enough that they can ignore the 'always' portion of a particular racial entry. Then again, that doesn't mean they won't suffer repercussions, whether social, supernatural, or otherwise!