Thanks for the comments everyone. I hope I'm offering a different viewpoint for others to think about, and as I know absolutely nothing about anything in any of the other threads on this forum yet, I may as well carry on posting in this one and see what happens!
QR Codes...
You don't need to use 3rd party online services to create QR Codes, you could use this:
http://chart.apis.google.com/chart?chs=250x250&cht=qr&choe=UTF-8&chl=http://your_url_of_choice_can_be_put_here.com
the 250x250 are px size and you can change them to any square size you like.
Just post that full url in your address bar and save the resulting image.
In my paticular niches, I join every forum and group related to the subject that I can find and use a qr code as an avatar. Some avatars are too small, but most are able to scan ok in any Android QR code scanner app. Linking directly to my app, or marketplace listings, or website, or whatever else I can think of. Simple and free.
A less targetted, but larger volume method is to use xrumer or similar software to set up multiple profiles on multiple forums (I'm talking hundreds or thousands), all using the QR code as an avatar. Don't link to any google, or important, web properties with this! Link them to an article, press release or video.
Another use is to save the QR Code image that I've created, stick it in my favourite image editing soft - I use fireworks - and customise all the black bits (make sure to use dark colours).
I then have an interesting looking image to submit to all the photo sharing sites, like flickr, photobucket, tweetpics, facebook and so on and so on. Even artists sites like deviant art. I max out the title, description and tags with my keywords.
A good free online service to auto submit images to multiple sites at the same time is: http://pixelpipe.com
A good free online service to manually submit images to multiple sites at the same time (unless you have an image rss feed) is http://ping.fm
Hope this helps,
Paul.