So as expected, WildFly 8.1 download works fine in JBDS 8.0.0.Beta2a B106 since WildFly 8.1 is fully supported there.
With JBDS 7.1.1 it's slightly different. JBDS 7.1.1 cannot add WildFly 8.1 using Runtime Detection and we rely on Runtime Detection to add the runtime after the server has been downloaded. So for JBDS 7.1.1 users, WildFly 8.1 can be downloaded, but they will then have to add the server manually using Server -> New (and ignore the warning about wrong version).

BTW, Rob Stryker has pointed out another concern:
If some future server will call for a new runtime type in JBT/JBDS, this could cause problems.
For example if EAP 7 is added to stacks.yaml with a new runtime type, it will still show up in JBDS 7.1.1 for download, but it will not work. To avoid this problem going forward, I created JBIDE-17523