Writers have very specific patterns with how they work. It is well documented that some of the greats used to have very peculiar OCD habits of sitting in a pentagram with candles under the 3rd moon if Jupiter while only using 2H leaded pencils on papyrus hand-made paper, written in virgin blood for the first draft.
In all seriousness, most creative people have their own goofy tendencies and prefer repetitious patterns over random ideals in creating a work environment.
However, some times the ideas pour into the frontal cortex and the perfect writing spot isn’t available. Sometimes, a writer has to just, well, write.
On the desktop and laptop (pc, Mac, Linux) use anything you can. Personal preference works best. Scrivener, Word, yWriter, GoogleDocs, it doesn’t matter. If you have a desktop or laptop, write your happiness wherever you can.
On tablets and phones, cloud-based services are your best friend. Anything tied to DropBox, box.net or iCloud will automatically backup and enable you to grab it everywhere else. WriteRoom, QuickOffice, Evernote, Lite Writer, WordPress, even an email to yourself will work if you save it to draft mode.
A quick note to jot down an idea for a plot idea or a new story idea is great for an easy way to cram some easy words in while waiting in limbo.
For example, I’m using WriteChain to keep track of .. Something, and mobile WordPress to write this article, on my iPhone.