Good summary of the likely causes of a second half slow-down via Calculated Risk:
1) less Federal stimulus spending in the 2nd half of 2010.
2) the end of the inventory correction.
3) more household saving leading to slower growth in personal consumption expenditures.
4) another downturn in housing (lower prices, less residential investment).
5) slowdown in China and Europe
6) cutbacks at the state and local level.
Read more here.