While it's very likely that there will be a Blu ray add-on for the Xbox 360, the new rumor started by Australian technology site, Smarthouse, doesn't carry much weight since they've done this in the past. Xbox fansite XF did a quick search and found that Smarthouse is the same site that reported to have confirmation of an internal HD DVD drive from a so called insider only a few months ago.
So while it's likely there will be a new add-on at some point. The credibility of Smarthouse is brought into question regarding the "insider" info. It appears to be more of a guess based on the links provided by XF. The evidence says a lot.