Compared to Velvia 100, here are my experiences with a range of digital cameras I have known. All the film was scanned using Nikon scanners (Super Coolscan 8000 and 9000), except for the 4x5 transparencies, which are scanned on an Epson V700 (definitely a disadvantage). Digital uprezzing was done using either a stepped technique in Photoshop (bicubic in 10% increments), or more recently, Genuine Fractals. The printer driver was never permitted to uprez - files always sent at 360 (Epson) or 600 (Canon iPF) DPI at print size. Digital always at base ISO (100 (Canons, D3x) or 200 in the case of Nikons other than the D3x)
Canon EOS D30 - yes, the OLD 3 mp model, NOT the 30D... - Lower resolution than 35mm scanned Velvia, noise is less than film grain, so can sometimes look as good overall. Don't try to print past 8x12!
Nikon D70 - Equivalent resolution to scanned 35mm Velvia - lower noise means that image is marginally better. Prints in the 11x16 range at most, makes a really GOOD 8x12.
Nikon D200 - Significantly higher resolution than scanned 35mm Velvia, lower resolution than scanned 6x6 cm (Hasselblad) Velvia. Prints 12x18 much more comfortably than 35mm (color) ever did.
Canon EOS 1DsmkII - medium format film quality - resolution close to scanned 6x6 cm Velvia, although the different aspect ratio makes it difficult to compare. Distinctly higher resolution than a 6x6 cm chrome cropped to 3x2 aspect ratio. Dynamic range exceeds Velvia by a substantial margin. Prints 16x24 quite reasonably, although I wouldn't push much past that.
Nikon D3x - well above conventional medium format film quality, but not large format - resolution easily comparable to or better than scanned 6x9 cm Velvia. Prints 24x36 very easily (I'm more comfortable with the D3x at 24x36 than I am with the 1DsII at 16x24). Extra dynamic range is very noticeable over any film.
4x5 film still exceeds any of these cameras for raw resolution (some of the high-end Phase backs may be in 4x5 territory), although 4x5 resolution versus D3x dynamic range would be an interesting choice. The higher camera resolution gets, the more near-perfect technique becomes important. The D3x produces 6x9 cm results, but ONLY if it is handled as a 6x9 (sturdy tripod, remote release, extremely careful focus). It is tempting to use as a 35mm camera, because it looks like one, and it will produce better than 35mm results used this way, but it isn't a 6x9 unless it is handled as a 6x9! This will be at least as true, if not more so, for any future DSLR with even higher resolution.
The other limit we are reaching is the size printer we're willing to live with. The best current DSLRs are already producing gallery quality 24 inch prints. The next step up requires a 44 inch printer and oversize mat board (a 24x36 inch print barely mats with a standard 32x40 inch board).
Is the next step really a higher resolution DSLR, or is it more choices at the resolutions we have? There is no reason why a 24.6 mp Leica M9 can't exist, or a 24.6 mp Nikon FM3d. If we have a wide range of cameras that can print 24x36 or larger, from models with 4 inch LCDs and tilt/shift sensors (technically possible if you get rid of the mirror) to a classic rangefinder that may even lack the image review LCD, and including SLRs with a wide range of features, is that not ultimately a more satisfying choice of tools than a few SLRs with even higher resolutions that require heavy tripods and 44-inch printers.
How about this range of cameras - all between 20 and 30 mp, optimized for prints up to 24x36 inches? All except the X1 are expected in the next year or two in some form or another.
Nikon D3xs (D3x with sensor cleaning, integrated GPS, integrated WiFi, integrated wireless flash transmitter) - everything anyone can fit into a big, heavy DSLR.
Nikon D700x (D700 body with a D3x inside - includes sensor cleaning and flash transmitter (less sophisticated than D3xs), but not GPS, WiFi)
Canon 1Ds mkIV (Canon's 29.3 mp answer to the D3x - lacks flash transmitter and GPS, but has HD video, which the D3xs lacks)
Canon 1D mkIV (20.1 mp, not quite full frame, but 12 FPS, with a 1080p60 video mode having FULL auto and manual control)
Canon EOS-3D (29.3 mp, no video, but 1D-level autofocus in a weatherproof body only slightly larger than a 5D)
Nikon S5 (Nikon's answer to all the rangefinder clamor - same old 24.6 mp sensor in a rangefinder body - unlike Leica, has AF if you want it)
Leica M9 (Ubiquitous 24.6 MP Sony sensor, although with 16-bit readout, custom Leica filter pack in an M body)
Sony Alpha 925 (Alpha 900 with 14 bit readout, improvements in AA filter and electronics to catch D3x image quality)
Sony Alpha 1000 (A true hybrid with as much attention paid to beyond-HD video quality as to stills - makes Jim Jannard see red)
RED Scarlet (Similar to the Alpha 1000, but started life as a movie camera, so optimized silghtly differently)
Nikon X1 (D3xs sensor in an EVF only camera with a 4 - inch XGA LCD on the back - sensor has 8 degrees of tilt capability and 18 mm of shift in any direction). The X1 uses its own line of lenses with increased image circle, although a F Mount adapter is available, which, of course, locks the shift.
This is (at least mostly) realistic based on what everybody's been known to do in the past (notice that the four Nikons ALL reuse the same sensor).