I thought I'd share my solution to battle that was fought today since I couldn't find any mention of it.

DotNetNuke Community Edition (06.02.00). Browsers: Chrome, IE8, Safari.
At ~2.00a (this morning), a successful login attempt was logged. As of ~3.00a all login attempts failed with this error:

System.Exception:
System.ArgumentOutOfRangeException: Non-negative number required.
Parameter name: count

Stack trace:
at System.IO.BinaryReader.ReadBytes(Int32 count)
at FiftyOne.Foundation.Mobile.Detection.Binary.Reader.ReadStrings(BinaryReader reader, Strings strings)
at FiftyOne.Foundation.Mobile.Detection.Binary.Reader.Add(Provider provider, BinaryReader reader)
at FiftyOne.Foundation.Mobile.Detection.Binary.Reader.Create(Stream stream)
at FiftyOne.Foundation.Mobile.Detection.Provider.get_EmbeddedProvider()
at FiftyOne.Foundation.Mobile.Detection.Factory.get_Instance()
at FiftyOne.Foundation.Mobile.Detection.Factory.Create(NameValueCollection headers, IDictionary currentCapabilities)
at FiftyOne.Foundation.Mobile.Detection.DetectorModule.OverrideCapabilities(HttpContext context)
at FiftyOne.Foundation.Mobile.Detection.DetectorModule.OnBeginRequest(Object sender, EventArgs e)
at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

I don't know the exact cause of the error, but I did find a suitable fix for our site. Since we're not really concerned with mobile browsers, I just removed the following line from the web.config:
<add name="Detector" type="FiftyOne.Foundation.Mobile.Detection.DetectorModule, FiftyOne.Foundation" preCondition="managedHandler" />

No upgrades or other changes occurred in that one hour of time when the site stopped working. The logs in this timeframe look normal, the only *even possibly* notable log would be that the site was crawled by a spider within that time. Other than that, one minute it was working, the next it wasn't.

This fix may cause other problems (I'll update this thread if I encounter anything related), but so far everything is looking good.