Just to add to what utnik said, there is a way of having top and bottom borders but it's not a fantastic solution.
Try this:
* Draw a Table where you want your text frame to be;
* When asked, set the table to be 1 row and 1 column;
* Alt-click the table;
* Go to the Line tab of the Properties Palette;
* Check Line at Top and Line at Bottom (nothing happens visually yet);
* Set the line Width to what you want;
* Go to the Colours tab of the PP;
* Set the Line colour to be what you want (now you should be able to see the lines);
* Alt-double-click the Table to enter some text and format it as you would a normal text frame.
It's not a particularly good solution and you don't have a huge amount of control over what happens but, for most simple needs, it's better than having multiple objects.
You can resize the table but make sure that you have the whole table selected, and not just the cell via Alt-click. Note that un-grouping the table will destroy the top/bottom cell borders when Scribus converts the cells in the table to normal text frames (which is really what they are but with an extra "trick" for the cell walls).
See the attached sample document for two simple examples.