Now you can keep a personal copy of the PDF on the CiteULike server. When you're logged in, just navigate down to any article with your web browser and you'll get the option to upload the PDF from your hard drive. After that, you'll have access to the content wherever you can log in to CiteULike.
There are obviously some restrictions to comply with copyright law. In order to prove that you have access rights to the content, you must be able to upload the PDF file directly into your account. That's to say, you must prove that you had a copy of the article in the first place - CiteULike will not be able to get hold if it in any other way.
Secondly, rather obviously, you must be logged in to your CiteULike account to download the article again when you, say, want to read it at home. That's the "personal" aspect. Think of this service as (a more organised and convenient) extension of your local hard drive. You can keep your content on it, but you can't use it to share that content with anyone else.
All data is backed up to a remote site and, in the initial testing phase, there are no usage quotas. I'll clearly need to impose some sort of limit at some point, but I've got plenty of disk space and I want to get an idea of popularity so I can work out where I should peg the limit.
Coming next (once I'm happy I've set the usage limits correctly) is the ability to do a full text search on all the PDFs in your collection. This will hopefully solve the "where did I read that?" problem - all you'll need to do is ask CiteULike to search through all the papers you've ever read and tell you exactly where you read it.