What type of freelance work do you do?
I do executive consulting and use the iPad pretty extensively for work.
For contracts, I use iWork Pages to update my "boiler plate" contracts (NDA's, Consulting Agreements, etc.). To review and sign contracts, I use Goodreader. I can make small changes and sign/mail completed agreements right at the client's site that way. (It helps to really "zoom-in" in Goodreader to get a clean signature.)
I do invoices in iWork Numbers. I also track various expenses, calls, and other things in Numbers. It's very convenient.
Oh, I use Apple's iWork.com cloud service with both of these to keep all of my documents in the cloud where I can access them from my iPad, iPhone, Laptop, or Desktop. It's easy to share documents in PDF and "MS Office" formats as well as get input on documents from the service.
For mockups, I make block diagrams by cutting / dragging various images from pages of Web UI, iOS UI, and system architecture clipping sheets. I arrange them in Keynote and/or SketchBook Pro.
For client notes I use MediaWiki, so we can collaborate together. I use Wiki Edit (blatant plug) to update the wiki from the iPad. For general notes and quick web clipping, Evernote's App is okay, but still doesn't support editing rich text on the iPad.
I also log into Cloud computing systems with iSSH (great app), and Rackspace Cloud's app.
The only thing I use "old style" computers for anymore is writing code. If Apple ever ports Xcode to the iPad I'll might just sell my Mac. LOL