You CAN edit fGDBs in QGIS (tested in 1.8.0 standalone and package based install, Win7 x64), however the technique to do this suggests that it is unintended, though I'm not sure of the reason for not enabling this feature. The steps to edit are described below:
- Load a fGDB layer
- Open the Layer properties and switch to the Fields tab
- You should see an edit mode toggle button that is enabled (unlike all the others in QGIS for this layer)
- Enable editing and start drawing / modifying
- To save your changes, the only way is to disable the editing mode using the same button as described above. You'll then be prompted to save.
Note that this only allows the editing of existing layers in the fGDB, I've not been able to add new layers yet.
Update
Expanding on @Vaiaro's answer, at the time of writing this answer originally there was only one driver for loading fGDB layers into QGIS. Subsequently the open FileGDB driver was reverse engineered and added to QGIS, and now this is the default that QGIS uses. The edit button should still be active if you load the fGDB layer using the FileGDB driver:
Choose Layer > Add Layer > Add Vector Layer... and then switch to the 'Directory' source type as shown below. If you have the ESRI FileGDB driver installed in your QGIS setup then you should see 'ESRI FileGDB' as an option in the type dropdown. FGDB Layers loaded using this driver will be editable (all the edit button instances should be visible and active).

If the ESRI FileGDB is not available as an option, then it will have to be installed via the OSGeo4W setup utility if using a package based install for QGIS (gdal-filegdb package), or by manually copying in the relevant files for a standalone version (see this answer for an older version of QGIS)
Insert, Delete and Edit the contents of simple datasets:. Upon seeing that, the inability to edit in QGIS is likely a limitation of the OGR driver.