Hi,
I am running Mac OS X Leopard version 10.5.7 on a MacBook3,1 with a wireless network using an AirPort Extreme. I was able to print wirelessly to my new Canon Pixma MX860, but my Macbook was unable to communicate with the MX860 scanner. Interestingly, though, the MX860 scanner could communicate just fine the other way with my Macbook because I was able to manually scan from the MX860 console and have scans wirelessly sent as PDFs to my Macbook.
Here's what I did to scan wirelessly to the MX860.
I used Easy Install from the accompanying Canon Pixma MX860 Setup CD-Rom disk *version U1.4* (purchased the MX860 on March 22, 2010) and followed all of the setup instructions in the Getting Started for Mac manual. In my case, I have a wireless LAN network with one access point, the Airport Extreme, but I would follow the instructions that are appropriate for your setup.
I went to the Canon MX860 driver page
http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=DownloadDet ailTabAct&fcategoryid=334&modelid=18121 and selected the MAC OS system files from the drop-down menu.
I downloaded:
_*MX860 series Scanner Driver Ver. 14.11.1*_ (Mac OS X 10.3/10.4/10.5/10.6)
mx860sosxsd14111ea8-2.dmg
(I didn't remember this file from the Easy Install, and I was unable to select this from the Navigator EX Scan menu, so I figured there was something wrong with it)
_*MP Navigator EX Ver. 2.1.2*_ (Mac OS X 10.2/10.3/10.4/10.5/10.6)
mpnexosx212ea8-2.dmg
(The version that came with the CD-Rom was an older version, 2.1.0, I think)
I installed both the Scanner Driver and the updated MP Navigator EX. The computer restarted after installing each program, so there were two restarts.
I opened the Solution Menu from the dock, chose Scan/Import Photos or Documents from the menu, and the Solution Menu opened MP Navigator EX (now version 2.1.2). From the MP Navigator Ex menu, I chose Scan/Import Photos/Documents (Platen), selected the Document Type, and voila, perfect wireless platen scans. When I checked the "Use the Scanner Driver" box, the Scanner Driver brought up more detailed scanning options. I was also able to scan a stack of documents using the ADF.
I know that this is a Snow Leopard thread, but I came across this thread when I did a search on exactly the same problem, so I wanted to post my solution in case it helps others. I think that the solution to your problem should be similar. If the two downloads specifically mentioned above do not help, perhaps try downloading and installing all of the other most up-to-date drivers/software on the Canon 860 page until something works.