I think it's important to have a purpose to work towards at any time during your day. This means: consciously deciding to work and then working, consciously deciding to relax and then relaxing, and in this case, consciously choosing to browse the web and then going and browsing.
This whole business of stealing a few minutes of browsing/planning/writing while waiting for the train or while being in the bathroom... it really doesn't amount to anything in your life. We all know that we can accomplish far more when we commit ourselves to one task for a long stretch of time rather than doing the task in chunks. In our case, web browsing is not even a task. It's not something you can finish. There's no end in sight. It's just something you do to spend your time while waiting, checking your email, forums, etc.
What's better for me is to have a book or an e-reader with me and read. With reading, at least you know you're getting something done and you know the end result (finishing the book). With browsing or planning, there's no end in sight. I would dedicate a separate hour just for planning or for browsing. If you really need to jot something down, that's where the portability of the iPod touch or iPhone comes in handy. It's always with you and in your pocket.
You'll also note that most of the time you web browse is to read. It's better to have a book because you don't waste time looking for what to read. You know what you're reading, you always have a purpose in mind.