Out of the mounts currently in the game, ~70 are buyable (with appropriate rep and/or currency) and 23 can be farmed from raids, instances, dailies, and holidays. 7 can be "achieved."
This does not include the mounts available from professions or class mounts.
3.1 brings us 4 new event mounts, 3 new horde mounts, 1 new flying mount, 1 fishing mount, and 2 achievement mounts to replace the 2 we're losing.
What does this mean for you, the player? Do you have to suddenly take up tailoring or engineering or even fishing to make your numbers?
Well, that's up to you.
I believe that Blizzard will increase the number of mounts available over time to make the 100 mount achievement more attainable without having to farm mounts that drop maybe once a year (1 try a week, 2% drop rate, 1 drop in 50, 52 weeks in a year).
The current problem isn't that there aren't enough mounts, because I expect more to arrive over time, just like they found a way to put in more pets with the Argent Tournament. The problem is that, right now, the people most favored to reach the 100 Mount Achievement are the unscrupulous.
Think about it. The best way to get this achievement for now is to cheat others. It will take forever to see one of the raid/instance mounts drop, even longer to see it and win the roll. The only way to guarantee your success and minimize your grinding time is to ninja the mount from everyone else.
Blizzard has created an environment around mounts where stealing is the most economical and sensible use of time management. People who hate fishing force themselves to fish, people who always made fun of holidays get involved. And people in instances maneuver themselves into the best possible position as Master Looter, sometimes as a sneaky ninja, sometimes with the more upfront "You can have anything except the mount if it drops."
Instead of mounts rewarding the people who have long loved tailoring, engineering, fishing, raiding, or holidays, the aficionados are imposed upon by others who hate being there, who are greedy and selfish and perfectly willing to complain to anyone who will listen, and who only talk about getting what they want.
The solution to all of this is not to make the farmable mounts easier to get. I'm against removing things forever, not against difficulty. The solution is that we need more easy mounts. Soon. The driving force here seems to be the achievement -- an achievement that is utterly impossible right now without winning multiple mounts from group activities. People need to be able to reach this achievement through nice, clean solo activities (yes, this includes professions) so that they rely less on group farming.
People enamored of this achievement, of collecting mounts, will get as many mounts as they can eventually. But I truly believe that having the achievement rely on rare drop mounts (as it does now) is a huge mistake and can only encourage people to act badly.
My solution is allowing the collectors enough choices so they can more-or-less solo the achievement. This way, the rare raid and instance drops are a bonus -- as Blizzard intended them to be.