I'm the developer of the GDAL PostIS Raster driver. I think that message comes from a really old version of GDAL. I think 1.7, or 1.8.
The message means that PostGIS Raster doesn't consider your loaded raster image as regularly blocked image. Let me clarify that:
A PostGIS Raster table is, basically, a bunch of raster tiles, stored in a PostgreSQL table, coming from a raster file from disk.
To load a raster file into PostgreSQL with PostGIS 2.0 extension, you need to use the PostGIS Raster loader. One of the most interesting loader's options is -t flag. It forces the loader to split your image into tiles. So, each resulting tile (plus its metadata) will be a PostgreSQL row. With this method, all the tiles have the same dimensions (width and height), and you can say the resulting PostGIS Raster table has regular blocking.
However, PostGIS Raster extension allows more raster arrangements. The above is the easiest case. The point is not all tiles stored in a PostGIS Raster table are forced to have the same dimensions. Neither the same pixel size. So, at the end, your PostGIS Raster table may be a bunch of unrelated tiles, with totally different dimensions.
The old versions of GDAL PostGIS Raster driver just were able to work with the first easier arrangement: raster with same size tiles. Even raster images without tiles (I mean, all the image in just one row) were considered as non-regularly blocked images. And the reading speed was really low, anyway.
The current version of the driver (packed with GDAL 1.10) is much faster, and allows irregularly blocked images. Could you please give it a try? And don't hesitate to ask me if you have more problems.