In SQL Server 2008R2 Will currently deployed applications still work if I turn on Mirroring without changing connection strings? I've googled a great deal, and can't come up with an answer to this.

In SQL Server 2008R2 will currently deployed applications (Asp.Net, .Net Smart Client) still work (with a connection string pointing to a single DB server) if I turn on Mirroring without redeploying the apps? 

The original apps are deployed and only expecting one server.

I understand the client apps without the "Failover Partner" section of the connection string won't know to use failover partner in the event of a primary failover event, however I don't want to break currently deployed apps. Rolling out new apps that are failover-aware will happen at one point, but would rather know the implications of setting up mirroring first.

Apologies if this should be on ServerFault as it pertains to .Net Application Access. Would ask in the Meta if my deadline allowed me the time.
Yes, it'll be just fine and you'll fail over just fine, provided that the server that was the single server is online.  If that server is totally down then without the connection string change the apps won't know how to find the other instance.

In short yes it'll work fine until you can get the connection strings updated.