Interesting find. I have only seen oi=spell rather than oi=stemming.
But I have noticed the "Did you mean..." suggestion showing at both the top and the bottom of page 1. Seeing a stemming suggestion only on page 2? That might be some kind of test. For me different stemming always seems to be integrated right into the original SERP.
My one frustration with "Did you mean..." is that sometimes I click on it and get NO results! So why bother to ask me, you know?
I've seen this too, initially with a search that included a generic word. The tip displayed at the bottom of page one:
Tip: These results do not include the word "[omitted word]". Show results that include "[omitted word]"
It seems that "stemming" is something of a misnomer in this context, since it's actually removing words from the search, rather than expanding them.
An additional variation is:
Tip: These results include the word "[expanded word]". Show results that include only "[original word]".
I've collected a few examples, which make for some interesting reading - in many cases, omitted words drastically alter the obvious user-intent of the query. I can't help but think that this is not behaving exactly as Google intends.
What seems to happen is a search query is translated into a closely related, more popular query. That has some pretty heavy implications!
This is also related to the recent thread breaking up uncommon words [webmasterworld.com], discussing some frustrations with Google keyword query expansion/contraction behaviours.