Use Spootnik ...

You can work online and offline using OmniFocus while Spootnik updates your Basecamp account in the background, keeping your mates and customers up-to-date. And Spootnik keeps your OmniFocus up-to-date about changes made inside Basecamp.

As your projects are shared over Basecamp you have as side-benefit the ability to use other Basecamp features like File Sharing, Message Boards, Time Tracking, ...

You are able to attach more than one Basecamp account to Spootnik.

This is very useful if you have customers maintaining different Basecamp accounts.

OmniFocus' built-in synchronization feature lets you share the whole database. Spootnik lets you share parts of your OmniFocus database using Basecamp as common ground. It lets you capture all tasks inside OmniFocus and share things which are of common interest (e.g. business stuff).

Do you know Blinksale, LessAccounting, Cashboard, Telescope, Timepost, ...?

They all integrate well with Basecamp and therefore now with OmniFocus, too. And there are a lot more out there! Most of them are listed here.

You don't need Basecamp but want to keep your OmniFocus database on your iPhone or/and Mac synchronized?

Despite Spootnik's biggest benefit is the synchronization with Basecamp, you don't need to attach a Basecamp account. You can use it as plain OmniFocus synchronization location like MobileMe.