If you're thrifty, the best balance of "cheap, portable, versatile" in a full power machine I know of is the Dell Vostro 1320. It's not even in the running for "light or thin" for a 13", but it's reasonably small, has a processor that doesn't require compromises for any kind of general use, has a built-in CD, gets decent batter life, and it's under $600 (slightly over with sales tax in some states.)

Ceases to be a good deal quickly if you need to push upgrades for better speed or battery life.

A couple of more general pieces of advice:
1) Decide on the size/form factor you want before looking at models. While most close sizes are basically interchangeable (11.6/12/12.1, 13/13.3, 14/14.1 and 15.4/15.6), this is the decision to make first.

2) Decide whether you like glossy or anti-glare screens, or don't care. Personally, I really detest glossy screens, and it has made buying on the consumer side tricky at points past.

3) If you're a heavy typist, check out each manufacturer's keyboard; going between Dell and Lenovo machines even at the same rough size tends to give me errors. Of course, dropping to a less than full-size keyboard (anything smaller than 13", and a few 13" models) can be even worse.

4) I wouldn't touch anything with a single-core processor today for real work; that said, the 1.3-4ghz dual core ULV processors seem to be an acceptable tradeoff for better battery life for many folks. About a 2ghz dual core seems to be a good baseline for general use, and IME the newer "Pentium Dual core" processors like the T4300 used in a lot of low end Dells are fine, although they don't have VT (virtualization) which is used by the Windows 7 "Windows XP mode" (which also requires Win 7 Pro.)

5) Lastly, the new Core i3/i5/i7 mobile processors are just starting to come out, and these are going to be a nice speedbump for many people. It might be worth waiting a bit for these to become more broadly available (although Dell has a couple of models orderable already.)