Most DELL servers, including their latest-generation R/T series, will run non-DELL drives just fine. There are a few exceptions, like the PowerVault MD3000, MD3000i, MD3200, MD3220 and some of their Blade servers. They absolutely require "certified" DELL drives with DELL firmware (no, you can NOT "flash" non-DELL drives with DELL firmware), or they will not function. Also, some DELL diagnostic and management tools will not recognize non-DELL drives, but this is rarely an impediment. DELL OpenManage will "flag" non-DELL drives with a non-critical error sign (that annoying yellow triangle with the exclamation mark), so if you're anal about having all green check marks, or you have OM set up to kick emails out even for non-critical errors, that could be modestly disturbing.
We have run literally every type and manufacturer of SAS and SATA drives in virtually every server that supports the interface. We run HP, IBM, and Sun drives, as well as straight "generic" drives -- including consumer-grade SATA -- in our DELL servers without any issues (aside from the niggles mentioned above). A 29xx will be running drives from either a PERC 5/6i or SAS 5/6i, both types of controllers have absolutely NO restrictions when it comes to SAS or SATA manufacturers or OEM. I'm running BOTH 2TB SAS drives pulled from an IBM ESXS storage array and 400GB 10K HP SAS drives in the same machine from the same PERC 6i controller, and they work just as well as any DELL drive. We have other R710's, and 2950's that run cheap, 2TB consumer-grade SATA drives that you can pick up from Newegg for a hundred bucks or less (sometimes). They work great, and do not require interposers.