Based on some documents on an IBM website, it looks like the 12.1" screen X61 ultraportable and tablet will be getting the new Centrino Duo (Santa Rosa) platform fairly soon.
Following are links to an IBM site that contains the announcement information for these upcoming notebooks:
According to the documents the "ivory paper release" (aka formal announcement) is set for May 31st and planned availability for June 2007. The only obvious changes based on available specs with either model is an upgrade to the Santa Rosa Intel platform. Meaning you'll get the following upgrades from the existing Lenovo ThinkPad X60 ultraportable series:

ThinkPad X60 Tablet PC (view large image)
It looks as though Lenovo will be using the Intel T7300 (2.0GHz) and T7500 (2.2GHz) in the X61 ultraportable, while they'll use the low voltage L7300 (1.4GHz) and L7500 (1.6GHz) in the X61 tablet pc. The good news is that they still plan to offer the SXGA+ in the X61 tablet:
"The ThinkPad X61 Tablet is a thin and light tablet with a 12.1-inch XGA TFT with 1024 x 768 resolution, a 12.1-inch TouchScreen XGA TFT with 1024 x 768 resolution, or a 12.1-inch SXGA+ TFT with 1400 x 1050 resolution. Models
weigh from 1.70 kg (3.76 lb) with the small footprint starting at 274 mm x 244 mm (10.8 x 9.6 in)."
Also some good news is that it appears in the X61 the option for 1GB of Intel Turbo Memory will be there, along with a 100GB 7200RPM hard drive.
Following is an example configuration for the X61 ultraportable:
And an example configuration for the X61 tablet pc ultraportable:
Thanks to Georgios for the tip!