I'm sounding off to confirm that its not a device issue. I've seen this happen often on my G2, and when I broke it and fell back to my Mytouch 3G it still happened. Now that I have the replacement G2 it still happens.
For me it occurs when commuting home in the greater SoCal region, I pass through an area of non HSPA coverage. In fact I lose coverage for seconds at a time (mountainous area). Once I return to an area of rich coverage its still not there.
I have discovered that it only affects data and not phone calls or text. I have received calls and text while the devices were not showing the E and H icon respectively. Its random for me, no telling when it will drop the data connection.