There is also this question—the only one tagged data-import which I suggest we retag etl. However perhaps I'm interpreting ETL too broadly and we should restrict to the specialised sense of Data Warehousing?
You can certainly use an ETL tool to populate something that isn't a data warehouse - for example I'm currently working on a process that de-duplicates data and populates a CRM system. I'd call it an ETL process. Looks like a duck, quacks like a duck.
I'd say etl belongs on processing with significant 'E' and 'T' (extraction and transformation) components. ETL is traditionally associated with data warehousing and the term was coined in that context. However, populating data warehouse systems is a long way from being he only application for ETL processes.