Jason – thanks for the reply. I turned off all plugins on my test site and turned on S2M in the network control panel for 6 sites (multisite). Here’s what happened with about 40k members – I tried it twice, one with a couple of popular plugins that are updated and pro and the second time
Wordpress admin panel: Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 80 bytes) in /home/mysiteacct/public_html/wp-includes/meta.php on line 567
[Tue Feb 07 21:54:07 2012] [error] [client 111.222.333.444] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 80 bytes) in /home/mysiteacct/public_html/wp-includes/meta.php on line 567, referer: http://mysite.com/wp-admin/network/plugins.php
[Tue Feb 07 21:52:17 2012] [error] [client 111.222.333.444] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 71 bytes) in /home/mysiteacct/public_html/wp-includes/meta.php on line 570, referer: http://mysite.com/wp-admin/network/plugins.php?error=true&plugin=s2member%2Fs2member.php&_error_nonce=d3da04cf9f
[Tue Feb 07 21:52:16 2012] [error] [client 111.222.333.444] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /home/mysiteacct/public_html/wp-includes/meta.php on line 567, referer: http://mysite.com/wp-admin/network/plugins.php?error=true&plugin=s2member%2Fs2member.php&_error_nonce=d3da04cf9f
[Tue Feb 07 21:51:16 2012] [error] [client 111.222.333.444] PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 32 bytes) in /home/mysiteacct/public_html/wp-includes/meta.php on line 559, referer: http://mysite.com/wp-admin/network/plugins.php