To answer your subject line, no, I'm not under the impression that Avalon is going to take away from BFL customers (at least not in any substantial way).
The way I see it, Avalon is great for people getting into ASIC on a smaller scale, maybe 1 or 2 units. They're more expensive, vastly larger than BFL units, and draw more power. I've got an order in to BFL for 9 Singles and a Jalapeno..and all of them will be able to sit on a single shelf (and cooled properly at that).
I could have placed the order for 9 Avalons, but that would require a 40U server rack to hold it all...not to mention I'm not entirely sure my house is laid out to supply 3600watts over an outlet (though I know zip about electrical engineering, so maybe someone could enlighten me here).
To that end, Avalon is still just offering a single product, compared to BFL's variety (which can tailor to a number of financial situations). And BFL accepts a few different pay methods as well

BFL currently has their estimates of shipping their first wave of products to customers February 18th. Avalon is currently (or at lest one) shipping their product to customers and delivering 65Ghash/sec per unit. They SAY they're shipping 12 per day, meaning that let's say they continue to ship between now and the 18th, that'd be 14 THash/sec added to the network before the first BFL box even leaves their facility. That would represent a 50% growth in network speed and (I'm guessing) a 50% increase in difficulty, which (I assume) would mean a 50% drop in income for BFL customers who were counting on a quick ROI assuming they'd be first to market.
ASIC mining will be profitable even after we break into the 50m difficulty range, and long afterwards as well. Adding 14TH/s to the network isn't actually very much when taking into account all ASIC orders from all companies.
Side car question: Is the assumption that the thousands (?) of ASIC miners will eventually just corner the market and split the 6 blocks per hour once GPU miners drop off due to decreased income? I couldn't find a thread that discussed.
ASIC miners will undoubtedly take over the market and split the 6 blocks...just as thousands of GPU owners do right now.
