The USA does not recognise the UK's Rehabilitation of Offenders Act, so no conviction can ever become spent when applying a visa. You must declare it to the Embassy.
Theft counts as 'moral turpitude'. (The USA's laws seem rather odd to most Brits. Simply nicking a Mars bar would count as moral turpitude, making it much more serious than beating someone up, which doesn't count as moral turpitude!). Under the US Immigration and Nationality Act, anyone convicted of an offence of moral turpitude (other than when a single offence was committed by a minor) is automatically barred from entering the USA. So, in the first instance, the Embassy will have no choice other than to refuse your visa application. However it can then be passed to Washington as an application for a 'waiver of permanent ineligibility'. The process takes several months. (The longest wait I've seen reported here was from someone who waited 15 months before being told that his visa application was rejected. He had two convictions for driving without insurance).
So it's unlikely that you'd get a visa for travel this summer. You might get one for travel later in the year but you could still be waiting at Christmas.
Since your visa application will involve you getting a Police Certificate, you should (if you've not already done so) make your application a.s.a.p:
http://london.usembassy.gov/pbcerts.html
Chris