I just received an email about this battery issue but had never experienced it (... to my knowledge, anyway. On the other hand, I'm not doing a lot of upgrades these days either). Apparently, Microsoft is looking into it:

After installing the Windows 7 upgrade, many customers have seen their machine's battery life dwindle significantly, even when working with a freshly charged battery. They've also been confronted with the Windows 7 warning message: "Consider replacing your battery. There is a problem with your battery, so your computer might shut down suddenly."

Ironically, Windows 7 was supposed to extend battery life on notebooks. During the Windows 7 beta, Microsoft said it has discovered that faulty drivers in Vista notebooks had prevented them from entering a quiet state, and that this caused Vista notebook batteries to drain faster than normal.

Microsoft confirmed the existence of the problem late last week, and the company will provide "information and guidance as it becomes available," a spokesperson said Monday in an e-mail.

It confirmed the existence of this problem? I guess so. Here's the official statement.

"We are investigating this issue in conjunction with our hardware partners. The warning received in Windows 7 uses firmware information to determine if battery replacement is needed. We are working with our partners to determine the root cause and will update with information and guidance as it becomes available."