One of Canada's best soccer writers (a shallow pool, I acknowledge), the Ottawa Citizen's Richard Starnes, nicely breaks down all the reasons why MLS expansion to Ottawa is unlikely.
He outlined some of the issues I did in June, while directly comparing the situation in Montreal and Vancouver with Ottawa's.
However, Starnes didn't bring up the possibility of a USL team. No one appears to be. That's a shame, because the environment for a solid USL franchise in Ottawa is there--especially if the Impact move to MLS (perhaps Montreal's USL team could just be shifted 90 minutes west).
USL is decent soccer. Another pro team in Canada will greatly benefit the domestic game and an Ottawa USL team could be a gateway to eventual inclusion in MLS (because it seems unlikely that MLS will actually stop at 18 teams).
If you love soccer and you live in the capital region you should start to make some USL noise. See the MLS expansion talk through to the fall when the league is expected to announce the next two teams, but realize what the odds are and be ready for the next step.
Update: globesports.com's Ben Knight voices similar sentiments: " before anyone seriously considers top-flight pro soccer on the leafy banks of the Rideau Canal, two essential things have to happen. Ottawa needs to land a second-tier franchise in USL-1 … and find dedicated, rich ownership."