Hi, prometheus.
I'm new to this forum too, I was diagnosed in december '07. I'm sure there are others that have been at this a bit longer, but I'll weigh in here. I've done a bit of gardening, and work as a landscape architect. I've been thinking about this myself. A couple of thoughts.
First on tools/aids. One thing I've seen that might be worthwhile is a small seat on wheels with a tray below for garden tools. There is also a "kneeler pad" with little handrails to use to get back up with. A.M. Leonard horticultural supply is a good place to look for this stuff- a couple of links below.
http://www.amleo.com/index/item.cgi?cmd=view&Words=ags1
http://www.amleo.com/index/item.cgi?cmd=view&Words=1786fg
I've also seen hand tools with longer handles, Am Leonard has those, too.
In terms of redesigning your garden, rasied beds would probably make things easier at some point. You might consider contacting the ASLA (American Society of Landscape Architects-
www.asla.org) to see if there is a local chapter nearby that would consider giving you some design input. Another option if there is a university nearby that has an LA program is to contact them. Sometimes they have community assistance programs that the students sign up for. This would be a great project for that.
A couple of other resources to consider. One is your closest land grant university / ag school. They generally have a coopertive extension service that has a bunch of gardening related info. Here's a link to one at Purdue University that is worth checking out-
http://www.ces.purdue.edu/vanderburgh/horticulture/links.htm#disabled
And lastly, if your state has a Master Gardener program, they are a great source of info, too.
Take care-
A07