You can transfer miles from United to Diners Club then to Delta, but you'll lose 50% of the value.

Obligatory Diners Club Plug

https://www.citibank.com/dinersus/mgm/apply/index.htm
Sponsor Code: SSKE4975

You also get 12,000 miles for signing up (1,000 per month that you make a charge, just make sure to buy some gas at a gas station that takes DC), and if you're nice enough to use my Sponsor Code above I'll get a couple thousand miles.