Half the argument for using cloud-based solutions like Google Apps to provide email is letting someone take care of your infrastructure and backup, but in larger business environments where stricter document retention policies apply and lawsuits could lurk around the corner the standard system isn't quite up to the task. If that's the situation you find yourself in, Google Apps Vault might be the answer.
Google Apps Vault provides a permanent searchable archive of all mail conversations and chats conducted through a company's Google Apps account. The search capabilities on a standard Gmail account are already pretty impressive; the main difference Vault makes is ensuring that crucial conversations aren't deleted and that you can search across an entire business.
Like Apps itself, Vault isn't free: you have to pay $5 per month per user (on top of your existing Google Apps fees). The Vault option went on sale in the US in late March, so we haven't been forced to wait too long. It's not an option individuals or very small businesses are likely to need, but for larger companies it could well be a sensible investment.
Google Apps Vault Brings Information Governance to Google Apps [Google Australia Blog]