Open skies as I understand it is not cabotage or foriegn ownership.
I think what would happen is ANY us carrier could fly to ANY european destination and back (as long as they can get gates, probably from an alliance partner) and vice versa. There would be no more artificial government quotas limiting arrival slots into certain cities.
This would not allow cabotage, which is a foriegn carrier arriving at a US city, embarking US pax, and then carrying those US pax to ANOTHER US city...essentially allowing foriegn carriers to serve the us domestic market. If this happens, all airlines would be based in some african country, repo to a us city, then start a 4 day trip (all for $1,300/year).
Most people think open skies would favor european airlines, allowing low-cost euro start-ups to undercut the lucrative international market between the us and europe.