An unfortunate weather event
The frost on the morning of May 9, 2010 has some corn  growers and those who advise them a bit nervous. The frost damaged  corn plants I looked at all had firm green tissue from the soil surface and below and should come through the frost well. The injury stopped at the soil line and after the recent rainfall the green tissue is now above the soil surface. The cold, wet weather after the frost added another level of stress which, by itself, might reduce stands slightly. There is some potential that these two situations may create a situation favorable for stand reducing pathogens. Warmer weather has arrived and frost injured corn plants will begin shooting new leaves soon. In fact, many fields have recently begun to improve in appearance. An idea of the extent of stand loss, if any, should be visible now.
Although emerged leaves were killed by frost, frost-injured plants will be advanced compared to later planted and unemerged counterparts. Whether or not yield was impacted depends on stand loss and issues slowing recovery, such as pathogens.
The frost damage will make corn stand evaluation more difficult. Keep in mind first leaves are likely to look abnormal as tips will be frost injured and the abnormal appearance does not mean an insect or disease problem. It is typical that some hybrids  are more susceptible to early season stress than others. Frost injury may be more severe on peat or sandy soils as these lose heat more quickly. Any replant decisions will need to be made in a very timely fashion.
Scouting early season corn
Corn scouting should begin in earnest as soon as fields can be rowed. Initial efforts should focus on evaluating stand. Determining the cause in areas with poor emergence  might require some detective work because there are many potential causes of poor stand. My suggestion is to start with the obvious. Was seed  planted? Most of us can make a mistake once in a while and any mechanical devise is predestined for failure.
Before blaming insects  (wireworms , seed corn maggot, cutworm ) or a disease , eliminate abiotic factors such as seed depth , compaction , drowning. Remember that dead seedlings, regardless of the cause, will rot under high moisture conditions. They also seem very attractive to wireworm. Cold, slow growing conditions do however; favor seedling-attacking insects and disease.
Look for corn seed. How about planting depth? Shallow-planted corn might emerge later and less evenly than corn planted a bit deeper. Shallow-planted corn (< 1½ inches) is exposed to greater temperature fluctuations and less consistent moisture. Shallow-seeded corn is also at risk for poor root development and root feeding insect damage when it does emerge.
Did the seed germinate? Lack of moisture or cold conditions are the primary cause of poor germination. Has the seed rotted? Fungicide seed treatments  do a good job of protecting seed and seedlings from some fungal pathogens but can be overwhelmed under prolonged very wet, cold conditions.
In the case of corn plants that had emerged and were frozen, check the below-ground tissue. If firm, the plant is likely to survive. Splitting the stems to look at tissue color will point out future problems. Brown, gray and water soaked tissue, particularly crowns, indicate a short life expectancy for the plant.
If plants are frost injured it is best to leave then recover on their own (particularly the small corn this season). Removing the dead tissue by mechanical means or spraying plant health inducing compounds will not improve the survivability of the crop and could make things worse.
Is there evidence of insect feeding? Seed corn maggot and wireworms are the two insects most often associated with corn emergence failures in southwest Minnesota. Seed corn beetles can also occasionally reduce stand. Slow emerging corn is at greater risk from these below ground pests. Unfortunately, there is no effective treatment, other than replanting, for these insects after corn is planted. The insecticide treated seed of with most Bt  hybrids (and others) should minimize problems from seed corn maggot but occasionally allow attack from heavy wireworm infestations.
Is replanting necessary? How bad is too bad?
Sometimes, things work out poorly. Decisions on replanting are seldom enjoyable when a significant stand loss occurs. These decisions are part science and part art. Common sense is at a premium. Yield potential from reduced stands must be weighed against potential yield reductions from later planting dates. Are missing plants still coming? Check for decay and below ground insect damage.
Injured and greatly delayed plants should be viewed with skepticism. Scattered, late-emerging plants will be out-competed by more vigorous neighbors and contribute little to yield. They may have a better chance in areas where most of the plants are in similar straits.
When replanting very thin to non-existent stands, seeding directly into the existing seed bed is a better option than working up mud. Existing plants should be removed by appropriate herbicide  or tillage  when replanting low but variable stands. Tillage may also be needed with fields hardened by heavy rain or previous tillage problems.
Evaluate fields carefully. While entire fields may require replanting, you may only need to spend the time, money and effort to replant a portion of the field, if any.