The below March 2006 blog lists ten examples.
http://googlesystem.blogspot.com/2006/03/google-calculator.html
I assume all the examples worked then.

Some do not seem to work now (August 2007).
By "not work", I mean doing a google search does not invoke the calculator on those.
Half of them (5) did not work on some of my trials.
Four of those I can't get to work at all.
"Half a cup in teaspoons" seems to work some times, but not always.
Try it on server 72.14.207.107 .

5+1 hasn't worked for me in the past

I have retried these. I now have some updated results and speculation as to what is happening.

I tried all ten at google.com, google.com/advanced_search, and IP 72.14.207.107.
Five of the ten examples invoke google calculator.
Two give the answer in a onebox from wiki. They apparently usurped the calculator link.
Changing the units in example 10 from kg to grams does not invoke calculator or wiki.
Another variation, multiplying the expression by one, does invoke the calculator.
Example 7 sometimes invokes a onebox with "Book results", but does not produce an answer.
My variation of the example, using "cup/2" in place of "Half a cup", invokes the calculator.
Example 2 does not have a onebox, but the top organic result is from wiki.
The snippet does not have the answer. Perhaps this will become a wiki onebox in the future.
Example 8 does not have a onebox. The top organic results are from "Yahoo! Answers".
The snippet does not have the answer. Perhaps the calculator result was usurped in preparation
for some kind of onebox in the future.

G – google.com
A – www.google.com/advanced_search
I – 72.14.207.107
c – Google calculator
w – wiki
(b) – Book results (sometimes)
x – only ordinary search results

........................................G.......A.......I.....notes
1 What's the answer to life, the universe and everything?
........................................c.......c.......c
2 How many seconds in a decade?
........................................c.......c.......c
3 Speed of light
........................................x.......x.......x.......[1]
4 17 to binary
........................................c.......c.......c
5 e^((pi)*i)+1
........................................c.......c.......c
6 10th root of 1024
........................................c.......c.......c
7 Half a cup in teaspoons
.......................................(b).....(b)......-.......[2]
   cup/2 in teaspoons
........................................c.......c.......c.......[5]
8 Ounces in a shot
........................................x.......x.......x.......[3]
9 When is Easter 2006?
........................................w.......w.......w
10 Earth mass in kg
........................................w.......w.......w
   Earth mass in grams
........................................x.......x.......x.......[4]
   1 * earth mass in kg
........................................c.......c.......c.......[5]

note [1]: The first organic result is from wiki but the snippet does
not have the answer.
note [2]: When I tried this in August 2007 the calculator was
sometimes being invoked. Now (September 2007) I am not
getting the calculator.
"Book results" are sometimes being shown, but the answer is
not shown.
note [3]: The first organic result is from "Yahoo! Answers" but the
snippet does not have the answer.
note [4]: I added this variation of example 10. Neither calculator or
wiki is invoked.
note [5]: I added these variations of examples 7 and 10.
The calculator is invoked.

Here are the search urls at google.com.

1. What's the answer to life, the universe and everything?
http://www.google.com/search?q=What%27s+the+answer+to+life%2C+the+universe+and+everything%3F

2. How many seconds in a decade?
http://www.google.com/search?q=How+many+seconds+in+a+decade%3F

3. Speed of light
http://www.google.com/search?q=Speed+of+light

4. 17 to binary
http://www.google.com/search?q=17+to+binary

5. e^((pi)*i)+1
http://www.google.com/search?q=e%5E%28%28pi%29*i%29%2B1

6. 10th root of 1024
http://www.google.com/search?q=10th+root+of+1024

7. Half a cup in teaspoons
http://www.google.com/search?q=Half+a+cup+in+teaspoons

   cup/2 in teaspoons
http://www.google.com/search?q=cup%2F2+in+teaspoons

8. Ounces in a shot
http://www.google.com/search?q=Ounces+in+a+shot

9. When is Easter 2006?
http://www.google.com/search?q=When+is+Easter+2006%3F

10. Earth mass in kg
http://www.google.com/search?q=Earth+mass+in+kg

   Earth mass in grams
http://www.google.com/search?q=Earth+mass+in+grams

   1 * earth mass in kg
http://www.google.com/search?q=1+*+earth+mass+in+kg

The Firefox 2.0 Search Box integrates with Google Suggest, which gives the answer right there in the search box (without needing to view the Google search result page).

For example, type 17+7 in the search box and Firefox adds =24.

It's worth noting that a search [Half a cup in teaspoons] does not invoke Google Calculator whereas [half a cup in teaspoons] does. Same goes for [Earth mass in grams] vs [earth mass in grams].

I've not tried any others, but I suspect Google Calculator is now fussy about case...

As Tony suggested, using a particular case can invoke the calculator in 9 of the 10 examples along with my variants.

Example 9 about Easter still seems to be problematic.
With some years the wiki onebox will give an answer.