When Luther met Zwingli to debate the nature of the Lord's Supper, before he sat down he chalked the words Hoc Est Corpus Meum on the tablecloth before Him. And so it is -- but it's still bread, and the wine is still the fruit of the vine.
UPDATE: Ebeth sent along a couple of links to Roman Catholic apologetics blogs. First, I'd say thanks to Ebeth for reading, then taking the time to share his (or her) concerns. However, this being a blog from a historic Reformed perspective, Ebeth will understand why I'm not publicizing a Catholic defense of transubstation here. John Calvin's Short Treatise on the Lord's Supper (1541) lays out the Reformed position -- basically, that Christ is truly present in spirit, but with bread and wine unchanged in their physical substance. Christ is only received in the spirit, not in physical consumption of the bread, though the two occur at the same moment.
More resources ancient and modern are posted on Monergism: The Church on the Threshold, here.