The FAQ 2008 says this about antigrav transports (paragraph 2.1.13, page 5) :

Q: What happens if I want to embark a Banshee unit in ruins (Dangerous Terrain) in a Wave Serpent (Skimmer unit) which begins and ends its movement in normal terrain but embarks the Banshee in Dangerous Terrain? Should I consider that the Skimmer has to stop in Dangerous Terrain while the Banshees are embarking and then roll a dice as per section 1.8.1 of the rulebook?
A: A Transport Skimmer will need to take a Dangerous Terrain test if it embarks units that are in Dangerous Terrain.

But now, what about for non antigrav vehicles when infantry is in buildings (impossible terrain for many transports) ?
As I understand the explanation of the FAQ, even if the transport is within 5cm of infantry, the transport is considered to reach infantry to embark it.
So an antigrav transport will have to pass a dangerous terrain test in infantry is in such terrain.
And what about non antigrav transports ? Do they also pass a dangerous terrain test ?
And what about infantry in building ? No test to pass as they can't enter the terrain ?

Well the FAQ speaks only about antigrav and dangerous terrain.
What about antigrav and impossible terrain ?

example : Dark Reapers in a building and Wave Serpent just behind. Can the Reapers embark the WS in a single move, or do they need to move first outside building and embark in a second move ?

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As I understand it either one or the other can move then embark.

If a formation doubles with infantry were too deep in buildings the infantry could move to embark on vehicles which forfeit there movement in the first move,  then the vehicles move in the second move and the infantry disembark forfeiting their movement. If you can get within 5cms the vehicles can just drive by and 'suck up' the infantry (whose moves are forfeit in both parts of the double move).

Hope that makes sense, and I'm right  

Embarking is done via the transport's movement, not the unit being picked up; so, if the Dark Reapers have to move to get to their transports, that's one move, so the Reapers will only be able to be picked up if the action is a double, not dropped off again.

From 1.7.5 Transport Vehicles Special Rule:

Transport vehicles can pick up and carry units as part of the transport vehicle’s move. The vehicle simply moves into base contact with the unit to be picked up, and then carries on with its move as normal. Note that the transported unit is not allowed to move themselves during the move when they are picked up.

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Wouah !
This gonna change radically Hojyn's and my game habits !
We used to consider it was the infantry unit which was embarking the transport before the transport moved away.

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A bit gamey (some may say) but correct, yet.  I do it like this anyway and my friends and i agree that that's how it always works anyway; why would  a Rhino go INTO a High Rise Tower Block to pick up the Marines, risking getting stuck and inexplicably blowing up (!?) when the marines can just walk down the stairs and hop on!
I feel it's more realistic this way.

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Yup. If the transport is part of the formation, 1.7.5 applies as others have pointed out, and the Transport must move over the unit being picked up. So if deep in a wood, rubble etc, the infantry take their #1st move getting into the open (while the transport skirt around the edge of the stuff), then in the #2nd move, the transport can pick up the infantry (and drop them off at the end of the #3rd move).

WE differ (rule 3.1.3 applies) as here the infantry must move to the WE, embarking at the end of their movement, at which point nothing moves until the following turn.

Skimmers are a little tricky as the way the rules and amendments are written are a bit ambiguous. They cannot be 'popped-up' while embarking / disembarking troops, so by inference they should be subject to the usual dangerous terrain tests of 'normal' ground vehicles. The way I envisage this is that the skimmer is flying below the height of treetops, buildings etc, so runs the risk of flying into some low level obstruction. However, it seems a bit daft to count rivers as dangerous terrain unless the skimmer has to stop on the river.

However, apart from this, skimmers must still conform with 1.7.5 if part of the formation, or 3.1.3 if a separate WE transport. Thus in the case cited, the Eldar Wave Serpents must move over the infantry, risking a DT test if they do so in 'Dangerous Terrain'.

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