Not at all. Metal miniatures are usually made from pewter, which is non-ferrous. In order to use magnetization for parts, you need to use magnets in addition to the parts (really small rare earth magnets aren't hard to get.) The big issue is that this often takes a bit of conversion work. An example in ABS plastic is that I was considering doing this with my Robotech Phalanx destroids. It would have entailed six magnets per miniature, one for each arm in both variants and two for the body of each destroid, as well as having to drill out each part enough to put the magnets in.
With neodymium magnets, they stay well attached - those things are amazingly strong for their size! They're dangerous to keep around small children and pets as if swallowed they can do quite a bit of harm to the intestinal tract - so if you get some keep the ones not used locked away.
Neodymium don't lose their magnetism if they aren't directly damaged or overheated. Maybe 1% in ten years. See FAQ here (and this site is great for buying them too): http://www.kjmagnetics.com/faq.asp The magnets you would use usually run between $0.10 and $0.25 each.
As ModernKiwi says, the neodymium magnets are very strong for their size. Magnetizing miniatures is very easy once you get the hang of it. The biggest problem will likely be gluing the magnets in the right way so they attract each other instead of repel. One turned around magnet can be a real pain in the ass.
I've magnetized a few Space Marines and some Tyrannid Warriors. Lately, I've even magnetized some Flames of War AT guns to swap out with different bases. One of the keys is to have a variety of magnet sizes. I've got 3/16" for potential big jobs, and I've used 1/8" for most of the Tyrannid stuff, and 1/16" for the smaller work (such as the 15mm FoW stuff). The 1/16ths can be hard to work with because they're tiny, but the end result is more than worth it.
And as IceShadow says, they are really inexpensive and easy to buy in bulk.