I rent trucks to people. Can I take the mileage deduction on these truck at tax time?

I guess you mean " Standard mileage". No; you can't ;if you use five or more cars at the same time (as in fleet operations) for your rental business purposes.
For further info, please viist teh IRS Web site; Publication 334 (2009), Tax Guide for Small Business

I rent my moving trucks to sub-contractors that complete the jobs. I charge them a daily rate. Can I use the standard mileage deduction on the trucks at tax time?

" I rent my moving trucks to sub-contractors that complete the jobs."-----> The rules are a bit more complicated if you use a vehicle that is titled in the name of your business (other than your sole proprietorship). If anyone ( your employees) uses the vehicle exclusively for business purposes, your business may take a deduction under the mileage method. However, the standard business mileage rate, as said above, may not be used for automobiles used when five or more automobiles are owned or leased and used simultaneously by the taxpayer (e.g., fleet operations). Rules providing for substantiation of an employee's ordinary and necessary expenses for local travel or transportation are also provided.