Started from last year, Oracle delivered quarterly PSU (PatchSet Update) containing CPU as well as several one-off patches. So far, the PSUs were cumulatives, no need to apply any of the previous ones to be able to install the latest one, like for the vast majority of the patches and CPUs by the past. And that was prety nice.
But, the PSU of July 2010 is quite different. Indeed, it has a prerequisite. The PSUJUL2010 must be installed on top of the PSUAPR2010. For instance, PSUJUL2010 for 10gR2 is 10.2.0.4.5 needs PSUAPR2010 - 10.2.0.4.4. It is quite unusual and I find it rather silently changed.
That does not mean the next PSU (in OCT2010) will have necessarily the prerequisite of the current one -PSUJUL2010, it will depend of the type of PSU : merge patch or overlay.
It must be kept in mind since it is very sensitive, from now, you must pay an additioanl and particular attention about the type of PSU. But if you missed some PSUs and want to install a new one, that could implies additional work.
Find out more in the note 854428.1 (6. Overlay Patchset Updates)