I have a beefy desktop sitting under a rarely used desk, and I VPN into that from a laptop. Thanks to VPN, the laptop is a cheap $350 model and there is no corporate data stored on it.

For a large corporation or government agency, I think the best infrastructure would be to have a massive virtual desktop farm (inhouse or on a hosted cloud) from which employees connect using laptops via VPN. When hiring a new employee or contractor, you just spin up a new VM using an image reconfigured based on their role. It's a lot easier to keep VMs backed up, and if your VM gets hosed after applying a service pack, then just re-image from backup.