BLAST results may now be shown using a new "TreeView" dendrogram, clustered on the basis of BLAST score. Either of two methods, "Fast Minimal Evolution" or "Neighbor Joining" can be used to construct the tree. To see a "TreeView", run a BLAST search from the NCBI BLAST page using the default 'pairwise' formatting option and click on the "TreeView" link on the results page. The NCBI BLAST page is found at: http://www.ncbi.nlm.nih.gov/BLAST/
For a fun sample sequence to BLAST, see http://blog.lib.umn.edu/messn006/blastit/
While BLAST results are highly heuristic and should not be construed to constitute an accurate sequence phylogeny, the graphic representation provided by TreeView may be helpful in analyzing similarity patterns in one's BLAST results.