1) confirm you have the proper firmware installed.
$ pkg_info | grep iwn
The installed version should be iwn-firmware-5.1, for OpenBSD 4.5-release.
(There is a more recent "5.1p0" which is used with -current. If you have 5.1p0 installed, pkg_delete it and pkg_add 5.1 instead, and see if that solves your problem.)
If that does not help:
2) post your problem on misc@, where there are a number of x200 laptop users. Your problem is specific to the 5100AGN; you are unlikely to get a solution to a firmware problem with that particular chip here ... though we have one OpenBSD user here with a 4965AGN ... There0. There0 has not been online here since May. I've sent a PM on your behalf; perhaps There0 will return and comment.