I'm trying to consume a georss feed from a C# project. Can I have queries against a #georss feed? For example 'nearest item to Lon/Lat' or 'items contained in bounding box X' .?
GeoRSS is purely a push service. It has no processing functions associated with it.
If you're using ArcObjects for instance, you could push the GeoRSS datq into a FeatureClass and then perform the operations you require through their tool box. Alternatively, there are third party API's that may be of use to you (Search this forum for open source tools).
Finally, without knowing what you're third party tools you may or may not be using, or where it is to fit in your work flow (i.e, is it a web service?, are you processing user queries from a GUI?, is it web/ASP.NET?), then my answer would be to write your own. If your using a mercator projection, then this is easy...others are a little more complicated, but you can Google the maths and even get an API that may suit your needs.
Not a great answer, but with a little more information this forum will produce the goods.
Think you need additional module like XQuery or JQuery
Here is a good example (MarkLogic API 4.2) using XQuery
If using Google Maps to display then GeoXML is using JQuery to do spatial queries