Newer yum versions fixed this behaviour, for example this is what I see on Fedora 9 x86_64 system:
# yum install ovzkernel
...
Setting up Install Process
Parsing package install arguments
Resolving Dependencies
--> Running transaction check
---> Package ovzkernel.x86_64 0:2.6.18-92.1.1.el5.028stab057.2 set to be installed
--> Finished Dependency Resolution
Dependencies Resolved
=============================================================================
Package Arch Version Repository Size
=============================================================================
Installing:
ovzkernel x86_64 2.6.18-92.1.1.el5.028stab057.2 openvz-kernel-rhel5 20 M
Transaction Summary
=============================================================================
Install 1 Package(s)
Update 0 Package(s)
Remove 0 Package(s)
Total download size: 20 M
Is this ok [y/N]:
I have fixed a note in wiki describing that one needs to specify architecture manually.