I think thats a weed. What sort of datura did you sow?
I would say so. My datura in a pot I increase from cuttings taken each year. The leaves are nothing like that.
I'm pretty sure that's the weed known as 'fat hen'. Don't let it seed as it produces 20,000 seeds per plant - if it did come in the compost that might explain why! See here:
good general weed identification page:
Heheh - it was just the best image I could find!