Just upgraded to Xcode 5.1, getting the following error:
ignoring file ...Dependencies/SalesforceNetworkSDK/libSalesforceNetworkSDK.a, missing req
According to apple's release note, see the following note point.
Note:
Be aware of the following architectures issues when opening your existing projects in Xcode 5.1:
So you've to set architecture as below to support libs architecture.

Reference from this post.
Update: From May 15, you've to take build from 5.1.1, see this post.