Exactly - it's main function is to trigger magnetron inside N900, so it must be strong enough. small earphones, even the cheapest 0.50$ ones, are best source of it, as it always contain neodyme magnets.

Personally, I preffer to "fix" it other way - by glue'ing magnet to N900 itself, as hard-cut off microSD on cover removal; irritates me as hell.
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Of course, all of this is to be scrapped, if he doesn't mean magnet inside cover, but part on kickstand leg itself. It's just a tiny piece of metal, but people often mistake it for magnet (as it show magnetic properties, for some time, due to long, direct contact with real magnet).

/Estel