Description of problem:

On my test installation there are _only_ x86_64 and noarch packages.
With the lastest rawhide update a try of 'yum update' causes:

Installing:
 compat-db               i386       4.3.29-2.fc7     development       1.9 M
     replacing  db4-devel.x86_64 4.3.29-9.fc6
......

Installing for dependencies:
 glibc                   i686       2.5.90-8         development       5.2 M
 libgcc                  i386       4.1.1-41         development        78 k
 libstdc++               i386       4.1.1-41         development       346 k

Bummer!  That is definitely not a desired result.  BTW -
compat-db.x86_64 is scheduled for an installation too.

OK, lets try the other way which luckily works:

# yum update 'db4*'
Updating:
 db4                     x86_64     4.5.20-3.fc7     development       1.1 M
 db4-devel               x86_64     4.5.20-3.fc7     development       2.4 M
Updating for dependencies:
 compat-db               x86_64     4.3.29-2.fc7     development       1.8 M

Oh!  No i386 packages and related dependencies this time!  And
following that with 'yum update' brings in all remaining packages
without any attempts to add anything for an unneeded architecture.

So, depending on how you are doing that effects are different.

Version-Release number of selected component (if applicable):
yum-3.0.1-2.fc7