When my wife wants a big blue dragon/butterfly crossbreed, well I help her get it. It's been a long and difficult process, but it's finally hers. As of October 11th, 2009, Tamaril of Bleeding Hollow is one of the very few with 101 mounts. It was a combination of time, luck and money to get to 100 mounts.
For those of you who want to try for 100 mounts, here are a few tips. First, make yourself a spreadsheet of every mount available and group them by their source, e.g. purchased with PvP tokens, gold, random drop, etc. Do the Argent Tournament dailies every day. This will get you to exalted with all 5 city factions so you can buy all the faction land mounts and also get you Champion's Seals to buy the Argent mounts. That alone, with the "no rep required" purchasable mounts will get you to 58 mounts.
When you are getting close to 100, be sure to farm the first 4 Anubisath Sentinels in the Temple of Ahn'Qiraj for the four Battle Tank mounts. Once my wife got to 96 mounts, she dropped Skinning to power level Tailoring and then Engineering so she could craft the 2 mounts from each profession. You can level one up, craft the mounts, add them to your mount inventory and then drop the profession to level another one. They will still count as mounts in your inventory for the achievement even if you can't actually ride them because you lack the skill in the appropriate profession.
I'm glad it's over so my wife can focus her energies (and mine) on gearing her death knight tank for Icecrown Citadel.