In 4-5-6 years you will be competing with new construction in Las Colinas especially with The Riverside and several other communities opening up.
The middle of the rental market is around $1200 or so, but there is a premium space to play in (as well as the sub-compact economy market). The main drawback is the variability in demand. You have a much smaller population to serve in the high-end market. However, with the large number of high-end foreclosure in many markets the demand for more premium rentals has increased. No one can say with certainty that that demand will stay up, though.
People who were paying $3-4k a month on a mortgage and got foreclosed are still looking for $2k or better homes to rent. They can't buy because their credit is hosed, but they do have reasonably high cash flow, albeit inconsistent.
Usually on the high-end rentals you will be looking for a reserve to self-insure against dry months on the renter. It is not unusual in these high-end cases to ask for $10k or more in deposit, and then grant them some leeway (fluctuation) in the balance, while you are drawing it down at a steady rate.
When the security deposit reserve account gets too low, you'll have to treat them like any other renter and start forcible detainer proceedings.
More consistent appreciation rates are one of the benefits of the higher end market, but estimating future trends is an exercise in futility. Realtors have a hard time predicting what will close this month -- these days, much less the trend 2-3 years ahead. The past has been pretty good for high-value areas, slightly above median market appreciation/deflation, but there are examples of values dropping.
Actually, rental rates are a bit higher in the areas you mention. For example, the one leased condo in Positano leased north of $4k/mo for 3/3.5 for one of their villa condo's at around 3,000 sq. ft.
There have been 2 recent rentals in La Villita at $1800 & $2,000/mo for 2/2.5 and around 1750 sq. ft. There are also 2 town homes currently on the market in La Villita asking around $2500/mo for about 2,000 sq. ft.
North American Properties at Positano sold their least expensive condo at a sales price of $229 when they broke ground in Fall 2006. That same town home to start in Summer 08 now is $265 . That should give you some idea of appreciation. There are only 170 homesites there, so competition for leases will be smaller there than La Villita.
Hope it helps.
LaVillita is a proven consistent attractor and it's at close-out with single family homes w/Darling and K. Hov. Portrait still has some new town homes for sale. When it finishes, you won't be competing against any of the builders. Positano is newer and it'll take a while for North American Properties to sell out. However, if you're planning on holding it for at least 5 years, they'll be sold out by then, too.
Positano also has a cabana w/a lounge and fitness center, pool/spa, sand volleyball, and BBQ grills. La Villita has the lake (however you can not swim, fish or do anything in it), greenbelt and then a pool and park coming. Both of these which would be very attractive to a young, active, social type tenant who tends to be the demographic moving to the area. The only amenity that Riverside will have is greenbelts, walking trails and possibly a park. In the Lakes of Las Colinas, there will be water features, but these are going to be north of $700k single family homes. Not your typical rental area.
La Villita has been around longer, so it has more ability to provide an appreciation factor. Positano doesn't really have that yet. Check out my blog below. No one has a crystal ball and can guarantee you a certain appreciation factor. However, based on past performance, the new communities coming to the area and the fact that Irving is supposed to grow to 5 million people by 2011 w/an average income of $109k p/year, nice appreciation looks like a good possibility.
We also just helped a client save more than $45k on his brand new town home in the Valley Ranch area. We work this area all the time and would be happy to provide any further info. you'd like.