Incorrect datetime indexing using Panels, potentially DataFrame too?
This is a very hard to diagnose bug as caching muddles its effects. I saw similar issues before, but not quite the same so I'm posting below. I'm not sure whether this affects DataFrames, but it seems likely.

``` python
from pandas import Panel, DataFrame
import numpy as np
import datetime
import pandas as pn
import pytz
est = pytz.timezone('US/Eastern')
dt1 = est.localize(datetime.datetime(2013,3,22,11,0,0))
dt2 = est.localize(datetime.datetime(2013,3,22,15,0,0))
dt1p = pn.Timestamp(dt1)
dt2p = pn.Timestamp(dt2)
df1 = DataFrame(np.arange(20).reshape(2,10) + .1, index = [dt1p, dt2p])
df2 = DataFrame(np.arange(20).reshape(2,10) + 20.1, index = [dt1p, dt2p])

panel = Panel({'a':df1,'b':df2}).transpose(1,2,0)
panel.ix[dt2,5,:] = 100

#the issue is that get_loc returns different locations even though they are equal by comparison
print panel.items.get_loc(dt2p)
print panel.items.get_loc(dt2)
print dt2==dt2p

#by merely printing it, the value is cached.
print panel[dt2]
#the following print statement returns the same value because dt2p == dt2
print panel[dt2p] 

#if the order is reversed, panel[dt2p] is output instead
panel = Panel({'a':df1,'b':df2}).transpose(1,2,0)
panel.ix[dt2,5,:] = 100
# the assignment of 100 is not there!
print panel[dt2p] 
print panel[dt2]
```

I am using python 2.7 on pandas 0.10.1

The most insiduous part is that if one tries to iterate over panel:

``` python
#print panel.ix[dt2, 5, :]
for t in panel:
    if t == dt2:
        print panel.ix[t,5,:]
```

and tries diagnose this issue by commenting/uncommenting the first print statement, the caching causes it to change the behavior in the loop! It almost caused me to believe there was a bug in the "print" statement.
