In a note to clients issued Thursday, Piper Jaffray’s Gene Munster raised both his Apple (AAPL) price target (to $390 from $371) and his estimate for 2011 iPad sales (to 21 million from 14.5 million). His key points:

• Increased Supply and Expanded Distribution Channels. After five months in which demand outstripped supply, Apple is now shipping iPads within 24 hours and rapidly adding distributors. By the end of September they will be for sale at all 1,000+ BestBuy (BBY) stores in the U.S. and according to Munster they are coming soon to Target (TGT).
• International Rollout and Adoption in Price Sensitive Markets. After launching the iPad in developed markets like Europe and the U.S., Apple is now moving into more price-sensitive countries where, as Munster puts it, “it is more likely that the iPad is used as a primary computing device for those who could not previously afford a Mac.”
• Uptake in Enterprise Sector. “Of any product Apple sells,” Munster writes, “we believe the iPad has the potential to garner the largest percentage of its sales from enterprise customers.”

By 2012, Munster estimates, tablets will have captured 17% of the PC/Netbook market. Apple essentially controls 100% of the tablet market today, but Munster expects at least 10 new tablets to hit the market before the end of the year, with devices running Google’s (GOOG) Android or Android 3.0 (codenamed Gingerbread) leading the pack. By the end of 2011, he expects Android devices to make up 26% of the  market compared with the iPad at 57% (see chart).

[Follow Philip Elmer-DeWitt on Twitter @philiped]