How to deal with callbacks?

Hello All,

I am currently checking flickr API. I can see there has been some work
done with flickr.rb etc... However it seems a bit out of date. I
basically want to do uploads. I need to provide callback support. Any
ideas how to implement such scenario:

my servers -> Client with form posting to flickr -> upload file to
flickr -> flickr calls back my server -> I greet the client with a
message "your file has been uploaded"...

Any help would be much appreciated.

/CArl