Infusing liquor with different flavors (seeds, herbs, spiced, fruits, cocoa nibs, etc.) is easier than you may think. All you need is an iSi Cream Whipper (re: nitrous oxide).
Here's what you do:
"Put room-temperature booze into the cream whipper. Add herbs, seeds, whatever. Close the whipper and charge it with nitrous oxide (N2O –the regular whipped cream chargers). Swirl gently 30 seconds and let stand 30 seconds more. Quickly vent the N2O out of the whipper, open it, and strain out the infusion. Done."
Step 1 Combine your booze and flavors.
Left: Mis en place --booze, parsley, whipper, N2O cartridge. Middle: pour in booze. Right: stuff in parsley.
Step 2 Add the nitrous.
Left: charge with N2O. Middle: swirl for 30 seconds then let sit for another 30. Right: vent the gas back to the atmosphere.
Step 3 Done!
You can find more detailed information at Cooking Issues.