Uhhh... What? If you mean that all your Spyderco's have steel that reacts to magnets, then, um, yeah... that would constitute a steel blade. That's what it does.
If you mean that you only have T-Mag Spyderco's, then that's... great; but still doesn't answer his question...
That magnet in the T-Mag is a neodymium "rare earth" magnet. This is the same kind as is used in Geomags, according to the Geomag website; although I would imagine that the magnet in the T-Mag is even more powerful. So, anything that your Geomag will mess up, the T-Mag will mess up, but probably to an even greater extent.
That only electronics that you can pretty much 100% safely put next to magnets is Flash Memory. So if you have a USB Thumb Drive, that would be safe to put in your pocket next to a magnet. But anything else, even something that's relatively safe next to magnets (like an iPod Nano), should be considered a no-no for magnet safety. Better safe than sorry.
Things you ned to especially watch out for are hard drives. So, if you have an iPod Video, for instance, KEEP THAT THING AWAY FROM MAGNETS!!!
Oh, I see you also asked how powerful the magnet is, and how far its field extends. Well, Neodymium magnets are around 35 times more powerful than a ceramic magnet of the same size, and the more powerful grades of Neodymium magnets can successfully hold up
1300 times their own weight!
However, their field only extends maybe an inch around them, and that would be dampened further by the knife handle. So, you'd probably be pretty hard pressed to accidentally wipe something out if you just walked past it. As long as you just kept things like electronics and credit cards in a different pocket from the T-Mag, you should be fine.