I'm actually not familiar with any cases that have specifically ruled on sub-domains. However, even invisible metatags have been held to cause "initial interest confusion" and thus were infringing uses of others' trademarks. (This theory is not without its critics, but the principle behind it seems sound).
If you have access to legal cases (I think these are available online), you might want to take a look at Brookfield Communications, Inc. v. West Coast Entertainment Corp., 174 F.3d 1036 (9th Cir. 1999), and Promatek Industries, Ltd. v. Equitrac Corp., 300 F.3d 808 (7th Cir. 2002). Those discuss metatag infringement and initial interest confusion. And again, while the whole metatag thing is a bit open to debate, I think that the logic in Brookfield is sound:
"[a]lthough there is no source confusion in the sense that consumers know they are patronizing [the competitor] rather than [the plaintiff], there is nevertheless initial interest confusion in the sense that, by using [the trademark] to divert people looking for [the plaintiff's] web site, [the competitor] improperly benefits from the good will that [the plaintiff] has developed in its mark."
Ah, wait, Jews for Jesus v. Google, Inc., 05-CV-10684 (SDNY 2005) involved Jews for Jesus suing google for the name of a blog, jewsforjesus.blogspot.com. However, I think the case settled.
Patmont Motor Werks v. Gateway Marine, 1997 U.S. Dist. LEXIS 20877 (N.D. Cal. Dec. 17, 1997), seems to contradict my analysis, as that case involved a domain
www.idiosync.com/goped -- and in that, the "goped" was just the "path" of the URL. Interactive Prods. Corp. v. a2z Mobile Office Solutions, Inc., 326 F.3d 687, 695 (6th Cir. 2003) was similarly decided.
However, I see a strong distinction between paths and portions of URLs. Nevertheless, a court might have held otherwise. I can do more research on it, but just not right now. It is a fascinating question, and I'll put up a corrected post if I find any decisions that hold otherwise.