I recall someone saying that if your freehub body is aluminum, the cassette will gouge into it. Can someone explain to me why this is a problem? I was thinking of getting some wheels from Mike Garcia with his speedcific hubs. Is an aluminum freehub a reason to buy some other hub?

When the gouges become so deep that they raise large burrs and/or the splines are deformed, it can become very difficult or impossible to remove or replace the cassette cogs. You saw this a lot when aluminum bodies first became available. But I think most manufacturers have fixed that problem by beefing up the splines. Still, you'll always gouge up an aluminum body to some degree over time - price you pay for cutting a few grams off the hub weight.

There are at least two things you can do to minimize the damage of cogs "cutting" into the aluminum freehub body. First, you can make very narrow aluminum shims from a soft drink can and insert a few of them in the gaps between the cogs and the freehub body. Second, you can use a cassette that does not use individual cogs (machined from a single piece of aluminum, for example) or you can use a SRAM cassette (e.g., PG-970) that joins the cogs together so that no individual cog is pressing harder into the freehub body. I am pretty sure I am not being as clear as I could be, so someone help me out here...

You will only have problems with the freehub body if you use a cassette with individual cogs. Both DA and Ultegra cassettes will work just fine on a alloy freehub body.

I like to save a few dollars and use 105 cassetes with my otherwise Ultegra 9-speed group. Does anyone know if 105 cassettes are different from Ultegra and DA?

The compatibility chart says 105 will work..

http://bike.shimano.com/publish/content/cycle/sac/us/en/technical_service/compatibility_charts/road_compatibility.-MainContent-0002-DownloadFile.tmp/Rear%20drive%20road%20compatibility.pdf.pdf

There are differences. You pay more for less weight and slightly better finish. DA cassettes have two or or three (can't remember how many) titanium cogs which wear out rapidly.

Edit: I'm almost certain (but willing to be corrected) that all Shimano cassettes are a combination of cogs on a carrier and individual cogs.