Icons:

Font used: Noto sans (http://www.fontsquirrel.com/fonts/noto-sans)
Key cap source code: http://all-free-download.com/free-vector/vector-clip-art/tango_preferences_desktop_keyboard_shortcuts_115846.html
Lisence: public domain

Additional editing: removed irrelevant keys using Inkscape, saved remaining key cap as pdf, imported into OmniGraffle, added letters, and finally exported as png with high resolution.

Text is 120 point, vertically centered, and then lifted using text offset Y -6%.

To convert the large-resolutio png with alpha to a useful iTunes art, do as follows:

$ convert images/Sami-icon3.png -background white -alpha remove -resize 1024x1024 images/Sami-icon-1024.png

To create the app icon, do as follows:

$ convert images/Sami-icon3.png -resize 120x120 images/Sami-icon-120.png