Excellent question! The only way for this to work would be to have strict definitions around the term unlimited. Netflix had unlimited DVD rental in the early days and this only worked because you couldn't get the next DVD until you had sent back the current DVD.

Unlimited may work for larger projects if:
- used a word to define a short task (could be task, job or a more creative name)
- behind the scenes have a set amount of time for a job
- work out the maths on your pricing per month, less profit needing to equal the maximum amount of hours that someone at X dollars per hour may be working. Like insurance, you could underestimate the hours a little with the thought that not everyone is going to send work at full capacity but this is a risk.
- when a client submits longer work, have someone split up the job into milestone jobs
- have these completed in a defined SLA timeframe i.e. "jobs are completed 12 hours following your previous job"

I hope this helps.