If you're willing to use a DS Emulator, you can download the ROM in about a week after the Japanese version is released.
For your actual DS, it's a little more work. Once you download it, get an R4DS card via Ebay, Amazon, etc, and with this blank DS Card, you can put ROMs onto it to play on your DS. With this ROM though, you'll need to add a translation Patch into the Rom's .rar / .zip file to make it English.
Once all of this is finally onto your R4DS card, you should be able to play a pirated Pokemon Black / White on your DS.
It's a vague answer, so google some of these terms if you need some more answers. :P
EDIT: If you're interested in these nifty specialty SD cards, then you can shop and compare DS cards here:
This will help you find the brand you need that best fits your computer / DS, and they start at about $7.00 US, I think (Free shipping too!), so it's pretty affordable.
EDIT2: ...Oh yeah, you can buy the Japanese copies at playasia.com, when they release. They're more expensive than past Pokemon games though ($65.00 +taxes/shipping), so be prepared to look in that couch for some quarters. :P