Fact is, the vast majority of solo tents are three-season models. Examples include Mountain Hardwear's Waypoint One ($195, www.mountainhardwear.com), Sierra Designs' Light Year CD ($140, www.sierradesigns.com), and Marmot's Eos 1 ($219, www.marmot.com). All weigh between two to three pounds, pack down very compactly, and provide plenty of shelter in wet and breezy conditions. The Eos and Light Year are traditional canopy/fly designs; the Waypoint One uses a single-wall design that employs a waterproof (and non-breathable) silicon-impregnated fly with a design that does a pretty good job of ventilating moist air out before it can condense. I have the two-person version and like it very much.
For three-season use, any of these would be fine. But for true winter camping they're probably not adequate. The canopy/fly models have just too much ventilation. And the Waypoint, while a fine tent, probably just isn't up to the pounding of a good winter storm.
So what to do? There are solid solo winter tents out there, but price becomes an issue. One I'd recommend is the Hilleberg Akto. It's ruggedly made and seals up nicely so that you're reasonably snug even in cold, nasty weather. And the weight is pretty astonishing—only two pounds fourteen ounces, thanks to the use of super light but still durable materials. The downside is the price—$345 (www.hilleberg.com). Bibler's I-Tent is a compact two-person tent that's fine for one. Sub four pounds and very rugged, but also an even pricier $549 (www.biblertents.com). Might want to butter the wife up before buying that one.
That said, I don't think you need to get separate summer and winter tents. The Hilleberg is fine for summer use; if it's real balmy at night you just do without the tent completely (provided bugs allow that).
More solo shelters reviewed in "Friendly Confines" from the September '04 issue of Outside.