Apple's virtual personal assistant, Siri, can do all sorts of wonderful things, including creating and updating Notes right on your iPhone. Dave Caolo of 52 Tiger breaks down the process:
To create a new note, tell Siri, “Create a new note,” “Make a new note” or something similar. You can give a note a title at the same time, for instance, “Make a new note packing list” or “Create a new note places to visit.” That becomes the first line of the note.
Updating an existing Note is just as easy.
It's not perfect, mind you. Siri doesn't always properly understand what you're saying (and sometimes has trouble connecting to the network to find out). Apple hasn't yet giving Siri equal access to all system apps yet either (i.e. it can read you SMS but not Notes). For quickly getting a bit of information entered into your iPhone, however, Siri can be ridiculously fast.
Hit the link below for Dave's complete workflow. (I use a similar process to quickly create Reminders.)
Source: 52 Tiger