Nintendo has confirmed a new Zelda game for GBA several times. In fact, it is confirmed again in the June issue of EGM in an interview with Miyamoto from the D.I.C.E. Summit a while back. Here's what he said:
"[Capcom] actually went into planning on this right after finishing [
Oracle of Seasons and
Oracle of Ages for Game Boy Color]. It will probably be something close to the graphics of
Four Swords and will use the same style of camera we've had throughout the
Zelda series. I think we'll be able to launch it within about a year or so."
As far as what the silhouette of Soul Calibur II Link from Nintendo Power is, I have no idea. It's obviously not referring to SCII, since they've already put pics of that in the magazine. It could be a new GC Zelda game, a spinoff, a new SSB game, or maybe something else entirely (or perhaps none of the above, and they're just messing with our heads by putting that silhouetted Link in there to begin with ).
The bottom line is that we don't know and may have to wait until E3 to find out. It's only two weeks away now, though!