A server normally responds with SYN ACK in a 3-way handshake.
What would happen if it just responded with ACK?
The client will wait for a SYN packet, after a while it will time-out.
The relevant RFC is http://www.faqs.org/rfcs/rfc793.html
The RFC allows for separate ACK and SYN by the server but notes that these may be combined into a single packet. In practice I believe all common implementations combine the server SYN and ACK.
Stevens† has a state-transition diagram. This suggests that the client will timeout to a CLOSED state and presumably will try to establish the connection afresh.
If either end receives data packets before they have reached a connection ESTABLISHED state, they will be in SYN_SENT or SYN_RCVD respectively and they will send a reset (RST) and (I think) move to a CLOSED or LISTEN state.
† ISBN 0-201--63346-9