Partially beaten by G-Flex, but my answers touch on some different points so I'll post them anyway.
Is inter-caste breeding possible/likely/rare/inevitable/preventable?
All breeding is inter-caste, since males and females are implemented as castes now. Breeding occurs between any caste with a MALE tag and any caste with a FEMALE tag. You can't give a caste both the MALE and FEMALE tag (the game ignores it if you try).
It can be used for a multitude of things, so can I, say, create a white tiger, which is virtually identical to the tiger, but is white, more valuable, more rare, and easily distinguishable from the orange and black kin?
Color: you can make the bodies arbitrarily different between castes, so yes.
More valuable: yes, higher petvalue for the caste.
More rare: no, biomes etc. can't be set at the caste level.
Easily distinguishable: I'm not sure what you mean. You can give each caste its own ASCII tile and creature graphics.
Things like different dog breeds: Will it be a possibility to have several different types of dogs, all of which are linked to the creature "dog", but vary in size, attack power, [trAINABLE] tag, value, ability to be liked/detested, color, presence of tail, etc etc?
You can do all of those except "ability to be detested" -- I think that's only done via VERMIN_HATEABLE, and you can't set vermin info at the caste level. However, the offspring of two castes does not depend on the castes of the parents, just on birth ratios for the general creature. The appearance of the castes will be influenced by the appearances of both parents, but you could still have a Doberman and a Great Dane giving birth to a chihuahua.
How will castes within your own fort come into play? I think I have seen the separation between male and female castes, but is it possible to make things like noble castes (parents were noble, now they will be), miner castes (parents were miners, must be a miner, but born with novice lever miner or something)?
Noble caste: you can set up entity positions (e.g. nobility) to only accept certain castes, but as above with dog breeds, you can't ensure that the children of nobility are born with the same caste as their parents.
Miner caste: not sure. You can't do the inheritance part, as above. When defining the caste itself, you can only set profession
names -- you can't lock them out of professions.
The current version's entity defs have a PERMITTED_JOB tag, but it's unclear whether the next version will allow you to use that on a caste-specific basis, like PERMITTED_JOB:MINER:FEMALE and PERMITTED_JOB:CARPENTER:ALL. Finally, will castes be part of a creatures description or something? In other words, can I just press "v" and be able to tell its caste from another?
For dwarves, I think it'll show up with a
male/female symbol as before. That's probably a default triggered by the presence of the MALE or FEMALE tag. You can probably override it with a specific caste symbol. I'm sure it'll also show up in the creature's descriptive paragraph.
I think what was meant was, in worldgen, will the game just use the size of the creature when it was young (and completely harmless), or when it was old (and completely unstoppable), or will it calculate the birth date and ongoing age of the creature and update its size accordingly for any given year?
Yeah, I understood that was what it meant, and the answer is yes, it will calculate age and the resulting size as you'd expect, unless there's a bug.