AJAX and RIA’s are rapidly redefining web interfaces, but they can be expensive and complex to implement.
A key consideration is to keep server side logic on the server. Background scripts should be written to require little to no user interaction, and should allow users to view their progress as they execute, and receive an email of the results when they complete.
The synchronization required to preserve states across accesses and sessions is probably not cost effective. Accepting that the web can initiate resource intensive background scripts, but not interact with them (other than possibly killing them) is a reasonable consideration.
Testing for a script in progress should be done to avoid redundant and potentially corruptive requests.