I was wondering if there is a way at all to use some of ArcGIS 10's extension into arc engine.
A good example would be if we could put Arc Hydro extension into ArcEngine.
You are able to access the supporting libaries; but you would need to build the UI all yourself. ArcEngine offers you the ability to create your own run-time apps using ESRI technology; but you have to assemble the general flow yourself.
You also need to make sure you handle the license needs properly with the extensions; yes the ability is there but again the UI and plumbing (pun-intended for hydro) needs to be built by you.
You can call some of the UI dialogs but building the state and handling the input-output will for sure not be fluid.