Is there a way to alias/anchor an array in YAML?

Your example is valid YAML (a convenient place to check is YPaste), but it's not defined what the merge does. Per the spec, a merge key can have a value:

1. A mapping, in which case it's merged into the parent mapping.
2. A sequence of mappings, in which case each is merged, one-by-one, into the parent mapping.

There's no way of merging sequences on YAML level.

You can, however, do this in code. Using the YAML from your second idea:

mobile:
  - *SAMMY

you'll get nested sequences - so flatten them! Assuming you have a mapping of such nested sequences:

data = YAML::load(File.open('test.yaml'))
data.each_pair { |key, value| value.flatten! }

(Of course, if you have a more complicated YAML file, and you don't want every sequence flattened (or they're not all sequences), you'll have to do some filtering.)

Closest solution I know of is this one:

sammy:
  - &SAMMY1
    public/javascripts/vendor/sammy.js
  - &SAMMY2
    public/javascripts/vendor/sammy*.js

mobile:
  - *SAMMY1
  - *SAMMY2
  - public/javascripts/something_else.js

Alternatively, as already suggested, flatten the nested lists in a code snippet.

Note: according to yaml-online-parser, your first suggestion is not a valid use of << (used to merge keys from two dictionaries. The anchor then has to point to another dictionary I believe.

If you want mobile to be equal to sammy, you can just do:

mobile: *SAMMY

However if you want mobile to contain other elements in addition to those in sammy, there's no way to do that in YAML to the best of my knowledge.