According to the documentation, we should be able to set the user agent string used by Titanium: http://docs.appcelerator.com/platform/latest/#!/api/Titanium-property-userAgent
iOS allows you to set the property value, but effectively ignores it. We use our own hard-coded value.
We should likely continue to use our own value internally for things like analytics/geolocation/etc, and use the user-supplied one for Ti.network.HTTPClient operations. Give that usage, we may want to even make Ti.userAgent read-only and have a userAgent property on Ti.Network.HTTPClient.