After the Firefox QR Code Plugin, there is now also a Wordpress QR Code Plugin. Very cool.

Download the Wordpress QR Plugin

Important!: Be aware however that the QR Code only gives access to an URL and that it doesn't resolve the problem of the mobile version of your blog. Your visitors will shy away to download the fat web version of your blog: if they don't have a flatrate it will cost them a lot of money to go on your 500KB web version start page.
Therefore, if you use a QR Code, direct it to a special mobile version of your blog. As a reference: i-mode pages cannot be more than 10KB per page. Personally I would go a bit higher, but it's important to keep that in mind.

PS:
If you use neither Firefox nor Wordpress to install the plugin, you can always use Kaywa's Moblog (free with ads), Feed2Mobile (free) or the QR Code Generator.