We have several tenants that last for years, and we have tenants occasionally that disappear after a month or two.

Assuming you are talking about homebuyers, in many areas it has felt like all the buyers disappeared right after the Homebuyers Tax Credit expired.In many of these same markets there are signs of life.I would recommend that you contact a local RE agent and ask them what they are experiencing.If you are talking about investors not buying... unless an investor is using cash it is getting very hard to get any deals funded through more traditional means.

.- It seems like these are more risky as a project, because built-in equity disappears when the old house is demolished, and new equity depends on the builder delivering on time and budget.

The agent said he needs to do some paperwork, and then he disappeared.

My buyers backed out (didn't disappear, simply backed out) after dragging the closing out for over 2 months.

I believe Mark is suggesting you take a commercial loan against at least 5 properties, pay off the traditional mortgages, and then they disappear from your 10 limit count.I posted asking questions about this earlier and my banker confirmed that it "works like that" this afternoon.

The point is even good people with the best intentions can disappear overnight.

I found plenty of abandoned houses with huge loans taken out against them and the owners "disappear"

This money doesn't just disappear.  

After going through the process of becoming a full time investor over the last ten years and watching hundreds if not thousands of new investors disappear during that time, often without buying even one property.