Well, it depends on your application. I do not expect to get a notebook or netbook performance of course. There are many possible embedded tasks where such a board may fit. For example for Amateur Radio APRS gateway to manage one radio and an Internet connection there are applications available written in C, Ruby or Python. My server written in Python runs fine on ASUS WL500 router with a 100MHz ARM and 32MByte RAM from USB stick on OPENWRT LINUX. OK, CPU laod is around 20% but who cares. Radio is connected via USB, no other hw is used.
A second hand ASUS router (first generation) is available for $49 including housing and PSU. Any hw which can do the same for better price and a bit higher performance appreciated. This is just one example, there can be many others. Well, you do need video output. What you need in connectivity:
- min 1x USB 2.0 (2x is better)
- 10 Mbit/s wired Ethernet
- 1x serial RS232 port
- 1x WiFi
- sound interface
- floating point arithmetic for math intensive tasks
Can Raspberry Pi used for such application? I think yes. Is it competive for $25? Definitely.