Hi Andy!
It depends... If you are using OS or adapter mirroring: yes. If you are
using TSM mirroring: not necessarily. About a week ago
(http://msgs.adsm.org/cgi-bin/get/adsm0204/1043.html) Stuart Ward received
the following message in his activity log:

04/18/2002 08:03:15   ANR0207E Page address mismatch detected on database
volume F:\ADSM_SERVER\DB\DB5.DSM, logical page 821282 (physical page
3378466); actual: -1.
04/18/2002 08:03:15   ANR0247I Database page 821280 successfully read from
an alternate copy on volume E:\ADSM_SERVER\DB\DB5COPY.DSM.

So, in this case, a corruption in one database files did not lead to
corruption on the mirror.
Kindest regards,
Eric van Loon
KLM Royal Dutch Airlines