How to install iPhone apps through OTA without iTunes interaction?
The Simulator does not use a bundled .ipa file; it uses the constituent application.  Furthermore, the Simulator runs an x86 version of the application with tweaked x86 versions of the iOS frameworks.  In other words, there is no good way to get an .ipa package to run in the Simulator.  The original developer could build for simulator then provide you with the relevant contents of ~/Library/Application\ Support/iPhone\ Simulator/User/Applications/ but that's really your only hope as far as I know.