My MacBook's fan speed was running as hard as it could run for a couple days after installing Mavericks. I did an SMC Reset the day after install and that did not cure the problem.
One issue I noticed was that Mavericks default setting in Mail for checking mail is "Automatically". I set "Check for new messages:" to "Every Minute" instead of "Automatically" and that fixed the fan speed problem. I am unsure what the setting "Automatically" does other than raise the workload for the CPU, which causes the fan to come on to cool the CPU. With Mail open, launch Activity Monitor and look at the %CPU value for Mail with "Automatically" selected and then set to "Every Minute". With "Automatically" selected the %CPU value for Mail was continuously running at values between 25% and 60%. With "Every Minute" selected, the %CPU has dropped to between 0.0% and less than 10%.
All is now well with my MacBook running Mavericks.
Changing from "automatically" to "every minute" did fix the fan problem. And mail's CUP has dropped quite low now, but periodically (ironically not "every minute") it does spike to over 100%. Something isn't right with the way Mavericks and Mail get along, to say nothing about the way Mavericks and Gmail get along!. Thanks, Joe, for the "every minute" tip. At least my Macbook Air is less noisy now!
As an epilogue to what I said before, I'll add that the day after I set Mail to check "Every Minute" I changed back to the "Automatically" setting and have left it there since. By going to the "Every Minute" setting for a relatively short amount of time and then back to "Automatically" seems to have cured the CPU usage and high fan speed caused by Mail.
By leaving the setting in "Automatically" mail servers that use "Push" for new messages allow Mail to received the new mail instantly rather than wait for the periodic time you set, i.e., "Every Minute" before receiving the Mail. Apple puishes mail and so does Yahoo and to my limited knowledge, Gmail does not.
I have noticed that there is a periodic spike created by mtmd as it takes "Snapshots" or does other maintenance for Time Machine. It is a temporary increase in CPU usage and subsides in a relatively short amount of time. If you do not use Time Machine, you may not see this spike.