Hi,
This may have been addressed already, and if it has I apologise, but I wasn't able to find a definite answer..
In D7 the node ID is based around an integer field meaning it can hold a value between 0 to 4294967295.. This means the limit to the number of items stored on a D7 system is a little under 4.3 billion.. Generally module creators that create their own entities also use this and so also impose the same limit..
In general CMS terms this is a huge number and likely more than any site would need but in "application" terms this number becomes far smaller.. Think of a system that tracks transactions where many thousands of items might be created daily or even hourly.. Or a logging system that stores the last 10 days worth of logs from somewhere that are being created and deleted constantly..
I just watched Dries's keynote speech in Sydney where he suggests building web applications and services around Drupal which I think is a great idea but then the number of records that Drupal needs to store must be able to scale..
So as a feature request I think Drupal would benefit from raising this limit significantly if not being made "unlimited"..