In the past I've written about a passive D-Star capable repeater using a couple Maxtracs.
This of course is less than ideal as it's a carrier activated system, left open to intermod and such.
I have also written about Satoshi's GMSK node adapter. There was a tid-bit about this in the July 2009, QST, Eclectic Technology column by Steve Ford.
In the QST article it was talking about using Satoshi's board as D-Star Simplex Hot Spot. Mark, KB9KHM developed some windows software so one can use the node adapter as a simplex node to talk back to the gateway server of other D-Star repeaters.
Most of the bugs have been worked out, and the official non-stripped down Satoshi, GMSK node adaptor board can be used to make or convert an existing repeater for D-Star, with the capability to talk back to gateway servers for interlinking.
I suggest taking a look at David, G4ULF's blog.
His blog is a running log of development of a D-Star repeater that links to the worldwide dplus network running on homebrew components and standard UHF FM rigs.
Duplex use of the node adapter used to require two 18F2550 PIC chips running his code, instead of the one. Now KB9KHM's HotSpot software can emulate that. So you can get by with a mini-hotspot board, and just one PIC.
Satoshi's project has caught a lot of attention (and some flack), because you can construct the node adapter for about $75. An Icom D-Star system; one radio module, RPC controller and gateway runs about $3,000.
If you build or convert your existing repeater system, you can also keep backwards compatibility for repeating analog.
All you really need are direct (varactor or varacap) drive to transmit the GMSK data, and a discriminator output for receive. (As pointed out for the simplex hot spot use, most 9600 baud "data ready" radios will work.)
The retrofitted repeater system should be tuned for maximum 3 KHz deviation for best bit error ratio.
If the idea of running Mark, KB9KHM's windows based "hotspot" linking software for a permanent repeater installation doesn't thrill you, never fear, Dextra is here! (This is a very close basis of G4ULF's project)