Non-US freelancers… do you work for any US publishers? If so, do they have an up-to-date W8-BEN form on file for you? This is the declaration that that you are not a US citizen/resident and are not liable for US income tax. If your client doesn't have one of these, the IRS can insist that US income tax is deducted from your payments at source.
You don't have to deal with the IRS in the United States, but you do need to ensure that each of your US-based clients holds a copy of this form for you, assuming you deal with them as an individual (sole trader) and not as a limited company or other trading entity.
You can download a W8-BEN here: http://www.irs.gov/pub/irs-pdf/fw8ben.pdf
And there are instructions on completing the form here: http://www.irs.gov/instructions/iw8ben/ch02.html
To be honest, the instructions are nearly as confusing as the form, but the form is actually pretty straightforward, as far as I can tell.
The only bit that looks a little tricky is Part I, Sections 5-7, which, after some fevered Googling, seems to go like this if you are a UK taxpayer.
- 5: N/A
- 6: The UTR (Unique Taxpayer Reference) you use in all your dealings with HMRC
- 7: Leave blank — this is for your client to add additional info when they submit the form.
Please note that I am not a financial adviser, and if you access to one, or if you use the services of an accountant, I would urge you to talk this through with them.