Phil, I too have struggled with this, but have a workable solution to use map units and a display a reasonable legend, until independent setting of legend line thickness is supported. You are using 1.7.4, so you can utilize this workaround, as it leverages the new symbology options.
Let's assume your map is in UTM (map units of meter) and you want your 'Roads' layer classified by type with a different symbol thickness in map units for relative display purposes (i.e. not real-world accuracy):
Road > Width
Highway > 20 m
Secondary > 10 m
Gravel > 5 m
For your Roads layer, you will need to make a new column in your data source of type real, with an appropriate size and precision for your use, lets call it 'ledgscale'. Edit your data and input the road width (in meters, as above) in the ledgscale column. There are many tools in QGIS for auto-updating your data relative to the road type column (Field Calculator, SQL, etc.).
The ledgscale column will scale your road type symbol's base width in map units.
Open the layer properties dialog for the Roads layer and go to Style, then choose to use New Symbology. Choose Categorized for symbol method, and set the Column to whatever your road type column is. Before clicking the Classify button, change the default symbol. Set the symbol's Unit to Map Units and the width to 1 (i.e. 1 meter, in this case). This will be used as a basis for the size in the legend later, and for scaling to your desired map units width for map features.
Click Classify to generate your road type symbols. Click the Advanced button and choose ledgscale under the Size scale field sub menu. This will scale your base symbol width by the ledgscale integer for each feature:
Highway -> (symbol width of n map units) * (ledgscale of 20) = output of legend at (n * 0.3 mm) and map feature at n * 20 map units width
Highway -> (1 meter) * 20 = legend of 0.3 mm and map feature of 20 meters
The 0.3 mm seems to be the default size for legend line widths (and other composer lines, like frames). This seems to be similar to the default of 0.26 mm for symbol lines.
You can do this technique with the other symbol methods, i.e. Single, Graduated, and Rule-Based. Rule-Based is the most flexible, albeit complicated.
Caveats: Changing the symbol's width beyond/below 1 (say to increase/decrease the size of the line in the legend) will affect that symbol's width on the map as well. You will also have to adjust the ledgscale number for all features that use that symbol.
This wacky workaround is how I have found to make it work, but coding QGIS to support fully independent control over legend line size is the correct solution.