The North American website reports that "due to extensive planning and testing with [Sony's] 3G partner", the SKU has been delayed until April 2012.
The website speculates that difficulties in reaching a pricing agreement with the 3G network provider could be the reason behind the hold-up.
Sony's yet to comment on the delay, but it's said customers will be informed of the change in plan imminently. It's unclear whether those holding pre-orders for the 3G version of the system will be given the option to switch a vanilla Wi-Fi device, but it would certainly be wise of retailers (and indeed Sony) to accommodate the switch.
The PlayStation Vita's due out this weekend in Japan, with a release set to follow in February for the rest of the world.