He said the Mainland, so the Virgin Islands would'nt count.
However, while the Southwest was purchased, it was not done so peacefully, but essentially after an unjustified war and by essentially holding a bullet to Mexicos head.
I was thinking that if the Russians manage to hold onto the territory until the gold rush, it'd result in something very similar to the Mexican-American War up on the tundra.
An influx of American and Canadian prospectors become dominant and try to set up their own "Alaska Republic" after clashing with the Russians - who will also be bringing in their own settlers to mine the gold. The immigrants seek annexation to America or Canada, but because the territory is so valuable and a part of a world power, it won't go as smoothly as OTL's coup in Hawaii and instead will result in war.
Edit: Thought a little bit more about this, and whether it pans out this way will depend on when gold is discovered in the region. If the PoD is an earlier gold rush - say, during the Civil War - that makes Russia decide to keep the territory, then the war and the open spaces left on the Great Plains will likely reduce the number of American settlers and it might become a Russian-Canadian conflict. If the PoD is America simply deciding the territory isn't worth it, then the gold rush might take place after the Civil War and resemble the situation seen above. If it takes place at the turn of the century as in OTL, then the Japanese could become involved if it degenerates into a wider war for Russian possessions in the Far East.
All of this would have produce large-scale butterflies that would affect the course of a WWI-analogue.