This is an issue with the version of GDALTools that shipped with the 1.8.0 branch. In fact, this issue was only recently fully fixed (at least it seems to be). On my Mac version of 1.8, which still references the 1.9.2 Kyngchaos GDAL framework install (you can have 1.9.x and 1.10.x installed at same time), the issue still occurs.
These two commits from this year address the issue (GDAL version check is now a direct numeric comparison):
There is a high likelihood that you might be able to just use those diffs to manually backport the fix to QGIS 1.8, but there is no guarantee as there might be other related commits I missed.
Unfortunately, you can not just download the latest GDALTools plugin and replace the one that shipped with 1.8 as there have been significant changes that limit the latest plugin to only running under QGIS >= 1.9.
My recommendation: use the latest nightly build for Mac instead of 1.8 for the latest features in data source manipulation functionality. You can have both the nightly and 1.8 running on your Mac at the same time (they use different preference files), As long as you do not overwrite your 1.8 project with one saved from the nightly, you can utilize the nightly's data source manipulation functionality, then load the results into your 1.8 project.