Well, you've got me confused. "Network Button"? Don't remember ever seeing one, with or without Osh's menu installed. How about a screenshot?
AHHHH! Wait, I think I know what you're experiencing, although it doesn't have anything to do with a Network Button. If you upgrade to WP3.1 and have had the WP Header hidden by checking Minimal Mode > hide header checkbox in Ozh's config screen, yes, the top bar that contains the Network Admin and Log Out links no longer shows. Uncheck that checkbox and it comes back.
If you have Ozh's plugin Network Activated, you have to temporarily rename it, so WP doesn't find it and auto-deactivates. This brings back the stock menus. Then get into the Network Admin screens, re-activate Network-wide and, before anything else, hit the "Settings" link available in his entry ON the plugins page. This will take you directly to his config screen where you can turn the Admin Menu back on.