Update January 24th, 2013: source "A Whole New Code Search"
The same query repo:OpeningDesign/OpeningDesign GET
now returns result!
The search index must have been updated when then new search engine has been deployed (elasticsearch.org)
Original answer (January 6th, 2013)
Considering that:
it seems to be an indexation issue.
After their recent fileservers issues, this could be a side-effect.
As Iven points out, GitHub support should be able to clarify this.
The support did clarify (07 January 2013):
We have stopped adding newly pushed code into our codesearch index.
The volume of code has outgrown our current search index, and we are working on moving to a more scalable search architecture.
I'm sorry for the annoyance.
We do not have an estimate for when this new search index will be up and running, but when it is ready a blog post will be published (GitHub blog)