You're confusing the type of memory that causes SMS/Voicemail to stop being received.
Apps run in RAM. RAM is managed automatically by the OS and clears out space when needed by unloading apps that currently aren't being used. RAM filling up will not cause the issue of SMS/Voicemail not being received.
The Internal Memory where apps are installed to (ROM) filling up is what causes the SMS/Voicemail issue. The Maintenance Release fixed the issue where Bluetooth was causing Internal Memory to fill up unnecessarily.
See
http://androidforums.com/sprint-htc-hero/13022-ram-rom-what.html for the definitions of terms used above
See
http://androidforums.com/sprint-htc...ues-official-fixes-updated-nov-24-2009-a.html for what issues were fixed with the updates.
You can currently still fill up Internal Memory by installing too many apps and cause SMS/Voicemails to not be received. See
http://androidforums.com/sprint-htc...issues-workarounds-updated-nov-23-2009-a.html