What applications are the actual "DAS clients"?
DAS applications generally take the form of graphical genome browsers, protein feature/structure browsers, and even gene information browsers.
For an example of how Ensembl acts as a DAS client for gene information browsing, see the "Gene DAS Report" section of this page: http://www.ensembl.org/Homo_sapiens/geneview?gene=ENSG00000165029
Here's an interesting review of EBI DAS clients: http://www.ebi.ac.uk/~rafael/das/index.php?display=clients
Are DAS/1 and DAS/2 designed to interoperate? For example, will I be able to use a DAS/2 client and a DAS/1 server?
As proof of this, a proxy adapter is being developed that will provide a DAS/2 interface around an existing DAS/1 server, allowing DAS/2 clients to interact with existing DAS/1 servers: http://lists.open-bio.org/pipermail/das2/2006-October/000867.html
To fully realize 1 <-> 2 interoperation, there would need to be a DAS/1 proxy adapter for DAS/2 servers, to permit DAS/1 clients to interact with DAS/2 servers. Whether such a utility will be needed or developed remains to be seen.