According to USA Today, Virginia Tech's new supercomputer cluster is composed of G5 Xserves running at 2.3 GHz.
The 2.3 GHz speed does seem curious though, for a few reasons:
First, no such 2.3 GHz Xserve is sold to the general public, but it is technically feasible. The Power Macs do use a 2.5 GHz G5 (although fairly hot), and IBM has already announced an IBM G5 supercomputer with 2.2 GHz G5 970FX chips, using its IBM eServer JS20 blade server system.
Second, although Apple already sells 2.0 and 2.5 GHz machines, they sell nothing in between. That's a pretty big gap to leave empty. This does make one wonder if Apple was hoarding 2.3 GHz chips for a specific purpose (like making supercomputers), or perhaps that Apple was simply underclocking 2.3 GHz capable chips for use as 2.0 GHz chips in Xserves (at a time when yields of high speed G5 chips were too low to support high volume 2.3 GHz sales).
Third, this might suggest an imminent release of 2.3 GHz G5 Xserves to the public. The G5 Xserve was first announced in January, and although only recently has Apple caught up with orders, it has already been 9 months since the announcement. One would expect a speedbump to the Xserve soon, possibly at or before MacWorld in January 2005, and 2.3 GHz would be the perfect speed for that announcement from both from the technical and marketing points of view.
Finally, back when the G5 2.0 GHz Xserves were first announced, Apple Canada had an advertisement which indicated the Xserves were at 2.3 GHz:
The ad was later corrected, but it does seem (especially if the USA Today article is correct) that Apple may have originally planned to release 2.3 GHz Xserves right from the outset, but had to change its plans even after some of the Xserve advertising material had already been completed.