After struggling with it for a good while, Maggie Nelson finally figured out the answer to her problems with Oracle, PDO and CLOBs - they're just not supported.
LOB support was added to PDO_OCI in PHP 5.1. This is really cool, however, it appears that the LOB support really means BLOB support. After much investigation and self-doubt (e.g. "what if we're using streams incorrectly?"), we found out that PDO_OCI does not currently support CLOBs (BLOBs only!).
The bug's already been documented and remains open, but Maggie encourages all of the Oracle developers out there with the fact that Chris Jones (of Oracle) knows about the issue and plans to correct it in the next release of the extension.