Thank you for your report. This issue is not related to any of the ESRI Web APIs (JavaScript, Silverlight, and FLEX) and we have forwarded this issue to the ArcGIS Server team and they are currently looking into this.
At the moment you can perform a spatial query on your layer and retrieve the attributes from the results sent back from the server, then look for the attribute keys of each Graphic object.
Another alternative solution would be not having the alias names defined for the fields in your service, but this might affect some use cases of your application; e.g. FeatureDataForm automatically shows alias names associated with each attribute of your layer, etc...
Hope this helps.