Yep, just a workaround so you can actually add the sub-domain. Unless they modify the DNS for the parent domain to also point to DreamHost, it will not affect their site at all.
Now this is an interesting question
Firstly, you can not guarantee that all the domains and sub-domains you host will share the one IP address, even if they are hosted on the same DreamHost server. For example, all my domains here at DreamHost resolve to different IP addresses and they are all on the same server. In fact, I have sub-domains that resolve to a different IP address than the parent domain.
Basically, you can not be sure what IP address the sub-domain will receive until you add it to the DreamHost system.
Now here is the interesting part...
Lets say you have added the sub-domain, you can not ping it to determine its IP address, because you have not yet configured the external DNS to resolve the sub-domain and you can't do that without knowing the IP address! A catch 22 situation.
One solution (untested) would be to create a free dreamhosters.com sub-domain and set it to mirror the real sub-domain. Pinging the dreamhosters.com sub-domain should give you the correct IP address. Something like the procedure outlined below;
Edit: On further reflection, it may be possible to use the IP address that your DreamHost server resolves to. Example; if your sub-domain is on the 'bixel' server, you could ping bixel.dreamhost.com to obtain the IP address. Once again, this is untested by me.
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