If you want to acquire a domain name which you think will not be renewed, it is still better to get in touch with the original or current registrant as otherwise you may be in competition with others to pick the name up when it comes out of the deletion cycle.
Most registrars will hold a name registration open for a period to allow the original registrant to renew. This expiry period can vary between a 2 and 45 days, depending on the registrar and how they process the expiry. Deletion itself takes 5 days. As you can see it can take from 7 to 50 days before a domain name becomes available to another new registrant.
Different registries have different rules regarding expiry and recycling, so it may be worthwhile checking on what rules are being applied.
.com, .net names are fairly easy to acquire following deletion.
.org have special restrictions.
.co.uk do not become available again as they are held by Nominet, the registry, on behalf of the original registrant although 'detagged' ie un registered. To acquire an expired .co.uk domain name it is therefore essential to liaise and get agreement with the original registrant and Nominet.
See also http://www.snapnames.com/deleteprocess.html