Not a chance of removing it.
Sulfuras MIGHT be altered as Ragnaros is being revamped, but not Thunderfury.
On topic, you will be able obtain/complete Thunderfury come Cataclysm. MC, BWL, and the guy in Silithus will be in tact. Current Elementium Ore that drops in BWL will be renamed "Elementuim Ingots" and current Elementium Bars will be renamed "Hardened Elementium Bars" to make rooom for Elementium being the new mining deposit in the expansion.
Neither TF or Hand of Ragnaros is being changed. Same of MC and Ragnaros as a boss in MC himself. Cataclysm has some major revamps sure, but unless specifically stated you shouldn't assume that one specific thing is changing or being removed.
As a safety precaution you should farm it NOW and every week if you really want to get it, the sooner the better remember that. Who knows what bugs Cataclysm will bring that would prevent you even temporarily from getting it, personally I've had other issues:
1. When WoTLK launched the rep gains were absolutely crap, you literally had to run MC for 2 years, every reset full clear to get Exalted. Only months later did they realize how stupid this was and made all rep gains 100% regardless of level.
2. Additional Instances Cannot Be Launched, it was impossible to enter this instance in 3.2 becaues heroics had priority and everyone was farming them.
I hope it's taken away.
Casters get their legendary for 3 months, then it's outright removed.
Melee get their legendary for 4 years, and nothing happens to it.
you can get TF in cata there is still the achive and a new guild achive