By Les Christie, staff writerJune 29, 2010: 10:03 AM ET

NEW YORK (CNNMoney.com) — Home prices rose 0.8% in April compared with March and were up 3.8% from a year ago, according to the S&P/Case-Shiller Home Price Index of 20 major housing markets.

That good news is tempered by a couple of factors. First, the one-year comparison was against a low-ebb mark. In April 2009, prices were just above a five-year low. Overall, prices are off 30% from their peak

Secondly, the improvement came during a time when the federal government was heavily subsidizing home sales through an $8,000 homebuyer’s tax credit. That credit is about to expire.

“Other housing data confirm the large impact, and likely near-future pullback, of the federal program,” said David Blitzer, a spokesman for Standard and Poor’s.

Once the tax credit fully expires, home prices are likely to take a beating, according to Pat Newport, a housing market analyst for IHS Global Insight.

“The housing glut and foreclosures will drive the national Case-Shiller index down another 6% to 8%, with prices bottoming in 2011,” he said.

The strongest rebound has been in California, where S&P tracks three major markets. San Francisco prices jumped 2.2% month-over-month and are up 18% year-over-year, more than any other city in the 20-city index.

San Diego prices rose 0.7% compared with March and 11.7% since April 2009. Los Angeles prices rose 7.8% over the past 12 months, and 0.7% in April.

The biggest loser over the past 12 months has been Las Vegas, down 8.5%. Prices rose there 0.3% there month-over-month.

Only two cities saw values fall during the month. Miami prices fell 0.8% for the month, which pushed the city into negative territory for the year at -0.5%. New York dropped 0.3% month-over-month and is off 1% year-over-year.