Stop reading this and go here: http://hydlaa.com/smf/index.php?topic=31734.0 !
In
this post it was revealed that some skills are not yet able to be trained, due to those skill not yet being implemented in-game, and that the
Players Guide listing of
Inactive Skills was even out-of-date. Given that this could lead to problems and frustration in game, with new players losing faith in the game when they put effort into training a skill and not seeing any improvement of their character in-game, I thought I'd start this post to keep a tab on which skills were able to be trained, and which were not.
The following lists skills by 'trainability'. If you know from in-game experience that a given skill either can be trained (i.e. have levelled-up a skill, and have seen actual effects from doing so in-game), or if you know for a certainty that a given skill is still inactive, post here and I'll edit this first post accordingly, to keep the list up to date.

[Last update: August 11th 2007]
Active Skills
(i.e. skills that can be trained, and have an effect in-game) Combat Skills
Knives & Daggers
Maces & Hammers
Swords
Axes
Melee
Light Armor
Medium Armor
Heavy Armor
Magic Skills
Azure Way
Blue Way
Brown Way
Crystal Way
Dark Way
Red Way
Job Skills
Axe Making
Blacksmithing
Knife Making
Metallurgy
Mining
Repair Armors
Repair Weapons
Shield Making
Sword Making
Various Skills
Inactive Skills
(i.e. skills that either cannot be trained, or that can be* but do not yet have any effect in-game) Combat Skills
Polearms & Spears
Ranged
Shield Handling
Magic Skills
Anti-Magic
Job Skills
Alchemy
Herbal
Baking
Cooking
Mace Making
Brewing
Masonry
Painting
Tailoring
Glassblowing
Fishing
Bow Making
Leatherworking
Gem Cutting
Pottery
Various Skills
Assassin Weapon
Backstab
Body Development
Climb
Find Traps
Hide in Shadows
Lockpicking
Pickpockets
Set Traps
Riding
Swimming
Empathy
Argan
Esteria
Lah'ar
Musical Instrument
[ Edit on 8-11-07: Added Repair Armors to working skills list. Thanks Natrina! ]