Google indicates 50,000 hits on English pages for _fazed by_ versus 26,600 for _phased by_. An original/eggcorn ratio of 1.88 is very high, and indeed the substitution is found frequently in journalistic writing. The Guardian’s _Corrections and clarifications_ columns offer several examples, such as this one, with a hint of exasperation:
> A preview of tonight’s episode of 24 on page 89 of the Guide, states, “She’s the only one not that phased by Jack, Tony and Gael’s secret plot …” The word (once again) should be fazed.
The Language Log post referenced above provides further information, such as Arnold Zwicky’s commentary:
> MWDEU has an entertaining entry on “faze, phase, feaze, feeze”, which notes that “phase” for “faze” is very common and remarks that it “is almost a century old now, and we are not especially hopeful that it will be phased out.” But they do recommend reserving “faze” to mean ‘daunt’. [..]
The American Heritage® Book of English Usage has an entry on it as well.
Edit, 2008-08-26 (CW): Added reference to and cites for “unphased”, which, as rightly pointed out by Ken Lakritz in the Eggcorn forums, were missing from this entry.