No, the E and D major chords should be minor, if you want their thirds to fit into the C major scale.
I think Gt has it right. If you want to follow that progression, it has the E and D have to be minor.
If you solo over it and phrase it both in a way that resolves to C then i think it could be. Otherwise some songs could not be in any key (Creep by Radiohead for example).
That progression is not diatonic to any key. If it resolves to C, you would likely still notate it as C major and write the appropriate notes as accidentals.
Someones knowledge of guitar companies spelling determines what amps you can own. Really smart people can own things like Framus because they sound like they might be spelled with a "y" but they aren't.
Your tonal center would still be C, but you're borrowing both D and E major from the key of A major. You can view it like a detour because you still have the IV-V-I movement with F-G-C, which means C major. If you think it sounds good, then make sure the melody takes those chords into account. You can create some pretty dramatic melodies with chord progressions that modulate to different keys so take advantage of it.

Edit: Yeah, Arch took care of it.
known as Jeff when it really matters
Last edited by titopuente at Jul 22, 2008,
Music does not have to use only seven tones. Chromatic tones are often used.

You would have to alter the notes used in a melody or solo over those chords, you wouldn't want to play a G note over E or F over D, but if that progression resolves to C, which it does, then it's in C major.