For some reason, I always assumed that the location couldn't be nailed down, but Saruman did a little detective work, matching the background motifs in the shot with similar motifs in an old tents photo of taken way back in November 2000.

We now believe this old photo - taken by a New Zealand paper - shows the Muster of Rohan at the Dunharrow encampment. You can read all about that report, here.

From the picture above, you'll note the motifs match pretty closely. Additionally, in the left corner of the New Line photo, there appears to be a reddish curtain which could match the gatefold cloth entrance to the tents.

The suit of armour certainly appears to have the nasal bar on the helmet that we've come to associate with the Rohirrim, but I don't think that alone can be regarded as conclusive. The armour hanging off a stand is something you could expect to see in a portable setting such as an encampment. Unfortunately, it's too fuzzy to nail it down as belonging to a specific character - such as Theoden.