According to an invite sent to Mashable, Facebook will be holding "a mobile event at Facebook HQ" next Wednesday at Facebook's offices in Palo Alto, California.
The event will almost certainly contain major announcements regarding Facebook and mobile. Immediate speculation should surround the rumored Facebook Phone, which the company told us last month it is not building. While there's a chance Facebook will announce its own branded smartphone (not unlike the Google Nexus One), we doubt this is the path Facebook will take.
We do think this announcement is likely a hardware or platform-related announcement, though. They wouldn't invite the Silicon Valley press corp for something as simple as an iPhone app update. Perhaps it's launching its own mobile OS?