i've been reading quite a bit of commentary about this and there are two camps of speculation:
1) she made the announcement on the evening before july 4th to disappear into the nation's biggest news hole and dodge a lot of media attention.
2) she timed the announcement for the opposite effect, to dominate what is otherwise a typically soft-news cycle.
interestingly i'm not sure either theory pans out. look at google trends:
1. 4th of july
2. nathan s hot dog eating contest
3. joey chestnut
4. wimbledon finals 2009
5. staples center michael jackson tickets
6. sarah palin federal indictment
7. independence day quotes
8. kobayashi
9. happy 4th of july myspace comments
10. declaration of independence text
she's beat out by july 4th, wimbledon, and two entries for a hotdog eating contest. theory #2 doesn't really pan out for her if that really was her intention. and as for theory #1, that doesn't really pan out either because the trended term in the top 10 is about federal indictment charges, which are only rumored in the most tenuous way, but the concept is still dominating the most popular search term containing her name.
and remember, this is google. even our moms know how to use google. this isn't like trended topics on daily kos.
also, for any non-american members who really want to know what 4th of july means in america, the full google trends list is wonderful:
http://www.google.com/trends/hottrends?sa=X
i see homemade ice cream, flag cake, and costco hours on there.